Polymer composition and a power cable comprising the polymer composition

ABSTRACT

A crosslinkable and crosslinked polymer composition containing a low density polyethylene (LDPE) homopolymer or low density polyethylene (LDPE) copolymer has excellent direct current (DC) electrical properties. The crosslinked polymer composition has an electrical conductivity of 150 fS/m or less, when measured at 70° C. and a 30 kV/mm mean electric field from a non-degassed, 1 mm thick plaque sample of the crosslinked polymer composition when measured according to a direct current (DC) conductivity method. A cable can be surrounded by at least one layer of the polymer composition.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

BACKGROUND OF THE INVENTION Field of the Invention

The invention concerns a polymer composition suitable for a layer of apower cable, the use of the polymer composition in a layer of a powercable, a power cable comprising the polymer composition and to a processfor producing the cable.

Description of the Related Art

Polyolefins produced in a high pressure (HP) process are widely used indemanding polymer applications wherein the polymers must meet highmechanical and/or electrical requirements. For instance in power cableapplications, particularly in medium voltage (MV) and especially in highvoltage (HV) and extra high voltage (EHV) cable applications theelectrical properties of the polymer composition has a significantimportance. Furthermore, the requirement for the electrical propertiesmay differ in different cable applications, as is the case betweenalternating current (AC) and direct current (DC) cable applications.

A typical power cable comprises a conductor surrounded, at least, by aninner semiconductive layer, an insulation layer and an outersemiconductive layer, in that order.

Space Charge

There is a fundamental difference between AC and DC with respect toelectrical field distribution in the cable. The electric field in an ACcable is easily calculated since it depends on one material propertyonly, namely the relative permittivity (the dielectric constant) withknown temperature dependence. The electric field will not influence thedielectric constant. On the other hand, the electric field in a DC cableis much more complex and depends on the conduction, trapping andbuild-up of electric charges, so called space charges, inside theinsulation. Space charges inside the insulation will distort theelectric field and may lead to points of very high electric stress,possibly that high that a dielectric failure will follow.

Preferably there should be no space charges present as it will make itpossible to easily design the cable as the electric field distributionin the insulation will be known. Normally space charges are locatedclose to the electrodes; charges of the same polarity as the nearbyelectrode are called homocharges, charges of opposite polarity arecalled heterocharges. The heterocharges will increase the electric fieldat this electrode, homocharges will instead reduce the electric field.

Electrical Conductivity

The DC electrical conductivity is an important material property e.g.for insulating materials for HV DC cables. First of all, the strongtemperature and electric field dependence of this property willinfluence the electric field distribution via space charge build-up asdescribed above. The second issue is the fact that heat will begenerated inside the insulation by the electric leakage current flowingbetween the inner and outer semiconductive layers. This leakage currentdepends on the electric field and the electrical conductivity ofinsulation. High conductivity of the insulating material can even leadto thermal runaway under high stress/high temperature conditions. Theconductivity must therefore be kept sufficiently low to avoid thermalrunaway.

Compressor Lubricants

HP process is typically operated at high pressures up to 4000 bar. Inknown HP reactor systems the starting monomer(s) need to be compressed(pressurised) before introduced to the actual high pressurepolymerisation reactor. Compressor lubricants are conventionally used inthe hyper-compressor(s) for cylinder lubrication to enable themechanically demanding compression step of starting monomer(s). It iswell known that small amounts of the lubricant normally leaks throughthe seals into the reactor and mixes with the monomer(s). In consequencethe reaction mixture contains traces (up to hundreds of ppm) of thecompressor lubricant during the actual polymerisation step of themonomer(s). These traces of compressor lubricants can have an effect onthe electrical properties of the final polymer.

As examples of commercial compressor lubricants e.g., polyalkyleneglycol (PAG): R—[C_(x)R_(y)H_(z)—O]_(n)—H, wherein R can be H orstraight chain or branched hydrocarbyl and x, y, x, n are independentintegers that can vary in a known manner, and lubricants based on amineral oil (by-product in the distillation of petroleum) can bementioned. Compressor lubricants which are based on mineral oils thatmeet the requirements set for the white mineral oil in EuropeanDirective 2002/72/EC, Annex V, for plastics used in food contact, areused e.g. for polymerising polymers especially for the food andpharmaceutical industry. Such mineral oil-based lubricants containusually lubricity additive(s) and may also contain other type ofadditive(s), such as antioxidants.

WO2009012041 of Dow discloses that in high pressure polymerisationprocess, wherein compressors are used for pressurising the reactants,i.e. one or more monomer(s), the compressor lubricant may have an effecton the properties of the polymerised polymer. The document describes theuse of a polyol polyether which comprises one or none hydroxylfunctionality as a compressor lubricant for preventing prematurecrosslinking particularly of silane-modified HP polyolefins.WO2009012092 of Dow discloses a composition which comprises a HP (i)polyolefin free of silane functionality and (ii) a hydrophobic polyetherpolyol of PAG type wherein at least 50% of its molecules comprise nomore than a single hydroxyl functionality. The component (ii) appears tooriginate from a compressor lubricant. The composition is i.a. for W&Capplications and is stated to reduce dielectrical losses in MV and HVpower cables, see page 2, paragraph 0006. In both applications it isstated that hydrophilic groups (e.g. hydroxyl groups) present in thecompressor lubricant can result in increased water uptake by the polymerwhich in turn can increase electrical losses or, respectively,pre-mature scorch, when the polymer is used as a cable layer material.The problems are solved by a specific PAG type of lubricant with reducedamount of hydroxyl functionalities.

There is a continuous need in the polymer field to find polymers whichare suitable for demanding polymer applications such as wire and cableapplications with high requirements and stringent regulations.

SUMMARY OF THE INVENTION Objects of the Invention

One of the objects of the present invention is to provide an alternativepolymer composition with highly advantageous properties for use in acable layer, preferably in a layer of an alternating current (AC) ordirect current (DC) cable, more preferably in a layer of a DC cable.

Furthermore the invention provides a use of an alternative polymercomposition in an insulation layer in a power cable, preferably to adirect current (DC) power cable.

The invention and further objects thereof are described and defined indetails below.

Description of the Invention

The present invention provides a crosslinkable polymer compositioncomprising a polyolefin, wherein the polymer composition comprises apolyolefin and a crosslinking agent, and wherein the polymer compositionhas an electrical conductivity of 150 fS/m or less, when measured at 70°C. and 30 kV/mm mean electric field from a non-degassed, 1 mm thickplaque sample consisting of a crosslinked polymer composition accordingto DC conductivity method (1) as described under “Determinationmethods”.

DETAILED DESCRIPTION OF THE INVENTION

Preferably the polymer composition is a crosslinked polymer composition,which, prior to crosslinking (i.e. before it is crosslinked), comprisesa polyolefin and a crosslinking agent, and which has an electricalconductivity of 150 fS/m or less, when measured at 70° C. and 30 kV/mmmean electric field from a non-degassed, 1 mm thick plaque sampleconsisting of a crosslinked polymer composition according to DCconductivity method (1) as described under “Determination methods”.

“Crosslinkable” means that the polymer composition can be crosslinkedusing a crosslinking agent(s) before the use in the end applicationthereof. Crosslinkable polymer composition comprises the polyolefin andthe crosslinking agent. It is preferred that the polyolefin of thepolymer composition is crosslinked. The crosslinking of the polymercomposition is carried out at least with the crosslinking agent of thepolymer composition of the invention. Moreover, the crosslinked polymercomposition or, respectively, the crosslinked polyolefin, is mostpreferably crosslinked via radical reaction with a free radicalgenerating agent. The crosslinked polymer composition has a typicalnetwork, i.a. interpolymer crosslinks (bridges), as well known in thefield. As evident for a skilled person, the crosslinked polymer can beand is defined herein with features that are present in the polymercomposition or polyolefin before or after the crosslinking, as stated orevident from the context. For instance the presence of the crosslinkingagent in the polymer composition or the type and compositional property,such as MFR, density and/or unsaturation degree, of the polyolefincomponent are defined, unless otherwise stated, before crosslinking, andthe features after the crosslinking are e.g. the electrical conductivityor crosslinking degree measured from the crosslinked polymercomposition.

The preferred crosslinking agent is a free radical generating agent(s),more preferably a peroxide(s).

Accordingly, the present preferable crosslinked polymer composition isobtainable by crosslinking with peroxide as defined above or below. Theexpressions “obtainable by crosslinking”, “crosslinked with” and“crosslinked polymer composition” are used herein interchangeably andmean that the crosslinking step provides a further technical feature tothe polymer composition as will be explained below.

The polymer composition of the invention is described below in view ofthe preferred crosslinked polymer composition embodiment, whichembodiment is also referred herein interchangeably as “Polymercomposition” or “polymer composition”.

The unexpectedly low electrical conductivity contributed by the polymercomposition is very advantageous for power cables, preferably for directcurrent (DC) power cables. The invention is particularly advantageousfor DC power cables.

The Polymer composition is preferably produced in a high pressure (HP)process. As well known, the high pressure reactor system typicallycomprises a compression zone for a) compressing one or more startingmonomer(s) in one or more compressor(s) which are also known ashyper-compressor(s), a polymerisation zone for b) polymerising themonomer(s) in one or more polymerisation reactor(s) and a recovery zonefor c) separating unreacted products in one or more separators and forrecovering the separated polymer. Moreover, the recovery zone of the HPreactor system typically comprises a mixing and pelletising section,such as pelletising extruder, after the separator(s), for recovering theseparated polymer in form of pellets. The process is described in moredetails below.

It has now surprisingly been found that when in a HP reactor system forcompressing the starting monomer(s) a compressor lubricant comprising amineral oil is used in compressors for cylinder lubrication, then theresulting polyolefin has highly advantageous electrical properties suchas reduced electrical conductivity which contributes to the excellentelectrical properties of the Cable. This is unexpected, since mineraloils are conventionally used for producing polymers for medical and foodindustry, wherein health aspects are of concern, not the reducedconductivity, as required for W&C applications.

Compressor lubricant means herein a lubricant used in compressor(s),i.e. in hypercompressor(s), for cylinder lubrication.

“Reduced” or “low” electrical conductivity as used hereininterchangeably mean that the value obtained from the DC conductivitymethod is low, i.e. reduced.

Preferably, polymer composition, comprises a polyolefin and acrosslinking agent, preferably peroxide, and the polyolefin isobtainable by a high pressure process comprising

(a) compressing one or more monomer(s) under pressure in a compressor,using a compressor lubricant for lubrication,

(b) polymerising a monomer optionally together with one or morecomonomer(s) in a polymerisation zone,

(c) separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone,

wherein in step a) the compressor lubricant comprises a mineral oil.

The resulting polymer composition has the above mentioned advantageousreduced electrical conductivity.

The expressions “obtainable by the process” or “produced by the process”are used herein interchangeably and mean the category “product byprocess”, i.e. that the product has a technical feature which is due tothe preparation process.

More preferably, the Polymer composition has an electrical conductivityof 150 fS/m or less, when measured at 70° C. and 30 kV/mm mean electricfield from a non-degassed, 1 mm thick plaque sample consisting of acrosslinked polymer composition according to DC conductivity method (1)as described under “Determination methods”; and

wherein the polyolefin of the Polymer composition is obtainable by ahigh pressure process comprising

(a) compressing one or more monomer(s) under pressure in a compressor,using a compressor lubricant for lubrication,

(b) polymerising a monomer optionally together with one or morecomonomer(s) in a polymerisation zone,

(c) separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone,

wherein in step a) the compressor lubricant comprises a mineral oil.

The low electrical conductivity of the Polymer composition is veryadvantageous i.a. in AC and DC power cables, preferably in directcurrent (DC) power cables, more preferably in low voltage (LV), mediumvoltage (MV), high voltage (HV) or extra high voltage (EHV) DC cables,more preferably in DC power cables operating at any voltages, preferablyat higher than 36 kV, such as HV or EHV DC power cables.

The invention is further directed to a crosslinked power cable,preferably to a crosslinked direct current (DC) power cable, comprisinga conductor surrounded by one or more layers, wherein at least one ofsaid layer(s) comprises a crosslinked polymer composition of theinvention comprising a polyolefin crosslinked using a crosslinkingagent, preferably peroxide. More preferably, the invention is directedto a crosslinked power cable, preferably to a crosslinked direct current(DC) power cable, more preferably to a crosslinked HV or EHV DC powercable, comprising a conductor surrounded by at least an innersemiconductive layer, an insulation layer and an outer semiconductivelayer, in that order, wherein at least one layer, preferably theinsulation layer, comprises a crosslinked polymer composition of theinvention comprising a polyolefin crosslinked with peroxide. Theexpression in the crosslinked cable of “polyolefin crosslinked withcrosslinking agent/peroxide” means that the polymer composition beforecrosslinking contains the polyolefin and crosslinking agent, preferablyperoxide.

It is evident to a skilled person that the cable can optionally compriseone or more other layer(s) comprising one or more screen(s), a jacketinglayer(s) or other protective layer(s), which layer(s) are conventionallyused in of W&C field.

The below preferable subgroups, properties and embodiments of thePolymer composition prior or after crosslinking apply equally andindependently to the polymer composition as such, as well as to thecrosslinkable cable and the crosslinked cable, as defined above orbelow.

In a preferable embodiment the Polymer composition has an electricalconductivity of 140 fS/m or less, preferably of 130 fS/m or less,preferably of 120 fS/m or less, preferably of 100 fS/m or less,preferably from 0.01 to 90 fS/m, more preferably from 0.05 to 90 fS/m,more preferably from 0.1 to 80 fS/m, more preferably from 0.5 to 75fS/m, when measured at 70° C. and 30 kV/mm mean electric field from anon-degassed, 1 mm thick plaque sample consisting of a crosslinkedpolymer composition according to DC conductivity method (1) as describedunder “Determination methods”. In this embodiment, the Polymercomposition has also preferably an electrical conductivity of 140 fS/mor less, preferably of 130 fS/m or less, preferably of 60 fS/m or less,preferably from 0.01 to 50 fS/m, more preferably from 0. 05 to 40 fS/m,more preferably from 0.1 to 30 fS/m, when measured at 70° C. and 30kV/mm mean electric field from a degassed plaque, 1 mm thick sampleconsisting of a crosslinked polymer composition according to DCconductivity method (1) as described under “Determination methods”.

The electrical conductivity of the Polymer composition is surprisinglylow even without removing the volatile by-products after crosslinking,i.e. without degassing. Accordingly, if desired the degassing stepduring the cable production can be shortened.

Further preferably, the Polymer composition has an electricalconductivity of 0.27 fS/m or less, preferably of 0.25 f/Sm or less, morepreferably from 0.001 to 0.23 fS/m, when measured at 20° C. and 40 kV/mmmean electric field from a degassed, 0.5 mm thick plaque sampleconsisting of a crosslinked polymer composition according to DCconductivity method (2) as described under “Determination methods”.

The cross-linking agent is preferably used in an amount of less than 10wt %, more preferably in an amount of between 0.2 to 8 wt %, still morepreferably in an amount of 0.2 to 3 wt %, with respect to the totalweight of the composition to be cross-linked.

More preferably, the cross-linking agent is preferably peroxide andbefore crosslinking the Polymer composition preferably comprisesperoxide in an amount of at least 35 mmol —O—O-/kg polymer composition.

The unit “mmol —O—O-/kg polymer composition” means herein the content(mmol) of a peroxide functional groups per kg polymer composition, whenmeasured from the polymer composition prior to crosslinking. E.g. the 35mmol —O—O-/kg polymer composition corresponds to 0.95 wt % of the wellknown dicumyl peroxide based on the total amount (100 wt %) of thepolymer composition.

Even more preferably, the Polymer composition prior to crosslinkingcomprises peroxide in an amount of 36 mmol —O—O-/kg polymer compositionor more, preferably from 37 to 90 mmol —O—O-/kg polymer composition,more preferably 37 to 75 mmol —O—O-/kg polymer composition.

For instance the concentration range from 37 to 90 mmol —O—O-/kg polymercomposition corresponds to 1.0 to 2.4 wt % of dicumyl peroxide (based onthe polymer composition) conventionally used for crosslinking of powercables. The peroxide content depends on the desired crosslinking level.Furthermore, the polyolefin may be unsaturated, whereby the peroxidecontent may depend on the unsaturation degree.

In one preferable embodiment the polymer composition is crosslinkedusing peroxide of at least 35 mmol —O—O-/kg polymer composition and hasa gel content of at least 30 wt %, preferably peroxide of 36 to 50 mmol—O—O-/kg polymer composition and has a gel content of at least 40 wt %,preferably of at least 50wt %, preferably of at least 60 wt %, morepreferably at least 65 wt %, when measured according to ASTM D 2765-01,method B, using decaline extraction.

Accordingly, the above electrical and crosslinking degree properties aremeasured from the polymer composition after crosslinking it using thecrosslinking agent present in the composition. The amount of thecrosslinking agent can vary. Preferably, in these test methods peroxideis used and the amount of peroxide is at least 35 mmol —O—O-/kg polymercomposition as defined above or in claims. The respective samplepreparation of the crosslinked polymer composition is described belowunder the “Determination methods”.

Peroxide is the preferred crosslinking agent. Non-limiting examples areorganic peroxides, such as di-tert-amylperoxide,2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne,2,5-di(tert-butylperoxy)-2,5-dimethylhexane, tert-butylcumylperoxide,di(tert-butyl)peroxide, dicumylperoxide,butyl-4,4-bis(tert-butylperoxy)-valerate,1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane,tert-butylperoxybenzoate, dibenzoylperoxide, bis(tertbutylperoxyisopropyl)benzene, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane,1,1-di(tert-butylperoxy)cyclohexane, 1,1-di(tert amylperoxy)cyclohexane,or any mixtures thereof. Preferably, the peroxide is selected from2,5-di(tert-butylperoxy)-2,5-dimethylhexane,di(tert-butylperoxyisopropyl)benzene, dicumylperoxide,tert-butylcumylperoxide, di(tert-butyl)peroxide, or mixtures thereof.Most preferably, the peroxide is dicumylperoxide.

In addition to crosslinking agent(s) the Polymer composition with theadvantageous electrical properties may comprise further component(s),such as further polymer component(s) and/or one or more additive(s). Asoptional additives the Polymer composition may contain antioxidant(s),stabiliser(s), water tree retardant additive(s), processing aid(s),scorch retarder(s), metal deactivator(s), crosslinking booster(s), flameretardant additive(s), acid or ion scavenger(s), inorganic filler(s),voltage stabilizer(s) or any mixtures thereof.

In a more preferable embodiment the Polymer composition comprises one ormore antioxidant(s) and optionally one or more scorch retarder(s) (SR).

As non-limiting examples of antioxidants e.g. sterically hindered orsemi-hindered phenols, aromatic amines, aliphatic sterically hinderedamines, organic phosphites or phosphonites, thio compounds, and mixturesthereof, can be mentioned. As non-limiting examples of thio compounds,for instance

1. sulphur containing phenolic antioxidant(s), preferably selected fromthiobisphenol(s), the most preferred being 4,4′-thiobis(2-tertbutyl-5-methylphenol) (CAS number: 96-69-5), 2,2′-thiobis(6-t-butyl-4-methylphenol), 4,4′-thiobis (2-methyl-6-t-butylphenol),thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, or4,6-bis (octylthiomethyl)-o-cresol (CAS: 110553-27-0) or derivativesthereof; or any mixtures thereof,

2. Other thio compounds like di-stearyl-thio-dipropionate or similarcompounds with various length on the carbon chains; or mixtures thereof,

3. or any mixtures of 1) and 2).

Group 1) above is the preferred antioxidant(s).

In this preferable embodiment the amount of an antioxidant is preferablyfrom 0.005 to 2.5 wt-% based on the weight of the Polymer composition.The antioxidant(s) are preferably added in an amount of 0.005 to 2.0wt-%, more preferably 0.01 to 1.5 wt-%, more preferably 0.03 to 0.8wt-%, even more preferably 0.04 to 0.8 wt-%, based on the weight of thePolymer composition.

In a further preferable embodiment the Polymer composition comprises atleast one or more antioxidant(s) and one or more scorch retarder(s).

The scorch retarder (SR) is well known additive type in the field andcan i.a. prevent premature crosslinking. As also known the SRs may alsocontribute to the unsaturation level of the polymer composition. Asexamples of scorch retarders, allyl compounds, such as dimers ofaromatic alpha-methyl alkenyl monomers, preferably2,4-di-phenyl-4-methyl-1-pentene, substituted or unsubstituteddiphenylethylenes, quinone derivatives, hydroquinone derivatives,monofunctional vinyl containing esters and ethers, monocyclichydrocarbons having at least two or more double bonds, or mixturesthereof, can be mentioned. Preferably, the amount of a scorch retarderis within the range of 0.005 to 2.0 wt.-%, more preferably within therange of 0.005 to 1.5 wt.-%, based on the weight of the Polymercomposition. Further preferred ranges are e.g. from 0.01 to 0.8 wt %,0.03 to 0.75 wt %, 0.03 to 0.70 wt %, or 0.04 to 0.60 wt %, based on theweight of the Polymer composition. Preferred SR of the Polymercomposition is 2,4-Diphenyl-4-methyl-1-pentene (CAS number 6362-80-7).

The invention is directed also to a process for producing acrosslinkable and crosslinked power cable, preferably a crosslinkableand crosslinked direct current (DC) power cable, as defined above orbelow, using the polymer composition of the invention.

The further preferable subgroups of the above properties, furtherproperties, variants and embodiments as defined above or below for thePolymer composition or for the components thereof apply similarly to themethod for reducing electrical conductivity, to the power cable,preferably to the DC power cable, of the invention.

Polyolefin Component

The following preferable embodiments, properties and subgroups of thepolyolefin component suitable for the Polymer composition aregeneralisable so that they can be used in any order or combination tofurther define the preferable embodiments of the Polymer composition.Moreover, it is evident that the given description applies to thepolyolefin before it is crosslinked.

The term polyolefin means both an olefin homopolymer and a copolymer ofan olefin with one or more comonomer(s). As well known “comonomer”refers to copolymerisable comonomer units.

The polyolefin can be any polyolefin, such as a conventional polyolefin,which is suitable as a polymer in a layer, preferably in an insulatinglayer, of an electrical cable, preferably of a power cable.

The polyolefin can be e.g. a commercially available polymer or can beprepared according to or analogously to known polymerization processdescribed in the chemical literature. More preferably the polyolefin isa polyethylene produced in a high pressure process, more preferably alow density polyethylene LDPE produced in a high pressure process. Themeaning of LDPE polymer is well known and documented in the literature.Although the term LDPE is an abbreviation for low density polyethylene,the term is understood not to limit the density range, but covers theLDPE-like HP polyethylenes with low, medium and higher densities. Theterm LDPE describes and distinguishes only the nature of HP polyethylenewith typical features, such as different branching architecture,compared to the PE produced in the presence of an olefin polymerisationcatalyst.

The LDPE as said polyolefin mean a low density homopolymer of ethylene(referred herein as LDPE homopolymer) or a low density copolymer ofethylene with one or more comonomer(s) (referred herein as LDPEcopolymer). The one or more comonomers of LDPE copolymer are preferablyselected from the polar comonomer(s), non-polar comonomer(s) or from amixture of the polar comonomer(s) and non-polar comonomer(s), as definedabove or below. Moreover, said LDPE homopolymer or LDPE copolymer assaid polyolefin may optionally be unsaturated.

As a polar comonomer for the LDPE copolymer as said polyolefin,comonomer(s) containing hydroxyl group(s), alkoxy group(s), carbonylgroup(s), carboxyl group(s), ether group(s) or ester group(s), or amixture thereof, can be used. More preferably, comonomer(s) containingcarboxyl and/or ester group(s) are used as said polar comonomer. Stillmore preferably, the polar comonomer(s) of LDPE copolymer is selectedfrom the groups of acrylate(s), methacrylate(s) or acetate(s), or anymixtures thereof. If present in said LDPE copolymer, the polarcomonomer(s) is preferably selected from the group of alkyl acrylates,alkyl methacrylates or vinyl acetate, or a mixture thereof. Furtherpreferably, said polar comonomers are selected from C₁- to C₆-alkylacrylates, C₁- to C₆-alkyl methacrylates or vinyl acetate. Still morepreferably, said polar LDPE copolymer is a copolymer of ethylene withC₁- to C₄-alkyl acrylate, such as methyl, ethyl, propyl or butylacrylate, or vinyl acetate, or any mixture thereof.

As the non-polar comonomer(s) for the LDPE copolymer as said polyolefin,comonomer(s) other than the above defined polar comonomers can be used.Preferably, the non-polar comonomers are other than comonomer(s)containing hydroxyl group(s), alkoxy group(s), carbonyl group(s),carboxyl group(s), ether group(s) or ester group(s). One group ofpreferable non-polar comonomer(s) comprise, preferably consist of,monounsaturated (═ one double bond) comonomer(s), preferably olefins,preferably alpha-olefin(s), more preferably C₃ to C₁₀ alpha-olefin(s),such as propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, styrene,1-octene, 1-nonene; polyunsaturated (═ more than one double bond)comonomer(s); a silane group containing comonomer(s); or any mixturesthereof. The polyunsaturated comonomer(s) are further described below inrelation to unsaturated LDPE copolymers.

If the LDPE polymer is a copolymer, it preferably comprises 0.001 to 50wt-%, more preferably 0.05 to 40 wt-%, still more preferably less than35 wt-%, still more preferably less than 30 wt.-%, more preferably lessthan 25 wt-%, of one or more comonomer(s). The Polymer composition,preferably the polyolefin component thereof, more preferably the LDPEpolymer, may optionally be unsaturated, i.e. the polymer composition,preferably the polyolefin, preferably the LDPE polymer, may comprisecarbon-carbon double bonds. The “unsaturated” means herein that thepolymer composition, preferably the polyolefin, contains carbon-carbondouble bonds/1000 carbon atoms in a total amount of at least 0.4/1000carbon atoms.

As well known the unsaturation can be provided to the Polymercomposition i.a. by means of the polyolefin, a low molecular weight (Mw)compound(s), such as crosslinking booster(s) or scorch retarderadditive(s), or any combinations thereof. The total amount of doublebonds means herein double bonds determined from the source(s) that areknown and deliberately added to contribute to the unsaturation. If twoor more above sources of double bonds are chosen to be used forproviding the unsaturation, then the total amount of double bonds in thePolymer composition means the sum of the double bonds present in thedouble-bond sources. It is evident that a characteristic model compoundfor calibration is used for each chosen source to enable thequantitative infrared (FTIR) determination.

Any double bond measurements are carried out prior to crosslinking.

If the Polymer composition is unsaturated prior to crosslinking, then itis preferred that the unsaturation originates at least from anunsaturated polyolefin component. More preferably, the unsaturatedpolyolefin is an unsaturated polyethylene, more preferably anunsaturated LDPE polymer, even more preferably an unsaturated LDPEhomopolymer or an unsaturated LDPE copolymer. When polyunsaturatedcomonomer(s) are present in the LDPE polymer as said unsaturatedpolyolefin, then the LDPE polymer is an unsaturated LDPE copolymer.

In a preferred embodiment the term “total amount of carbon-carbon doublebonds” is defined from the unsaturated polyolefin, and refers, if nototherwise specified, to the combined amount of double bonds whichoriginate from vinyl groups, vinylidene groups and trans-vinylenegroups, if present. Naturally the polyolefin does not necessarilycontain all the above three types of double bonds. However, any of thethree types, when present, is calculated to the “total amount ofcarbon-carbon double bonds”. The amount of each type of double bond ismeasured as indicated under “Determination methods”.

If an LDPE homopolymer is unsaturated, then the unsaturation can beprovided e.g. by a chain transfer agent (CTA), such as propylene, and/orby polymerization conditions. If an LDPE copolymer is unsaturated, thenthe unsaturation can be provided by one or more of the following means:by a chain transfer agent (CTA), by one or more polyunsaturatedcomonomer(s) or by polymerisation conditions. It is well known thatselected polymerisation conditions such as peak temperatures andpressure, can have an influence on the unsaturation level. In case of anunsaturated LDPE copolymer, it is preferably an unsaturated LDPEcopolymer of ethylene with at least one polyunsaturated comonomer, andoptionally with other comonomer(s), such as polar comonomer(s) which ispreferably selected from acrylate or acetate comonomer(s). Morepreferably an unsaturated LDPE copolymer is an unsaturated LDPEcopolymer of ethylene with at least polyunsaturated comonomer(s).

The polyunsaturated comonomers suitable for the unsaturated polyolefinpreferably consist of a straight carbon chain with at least 8 carbonatoms and at least 4 carbons between the non-conjugated double bonds, ofwhich at least one is terminal, more preferably, said polyunsaturatedcomonomer is a diene, preferably a diene which comprises at least eightcarbon atoms, the first carbon-carbon double bond being terminal and thesecond carbon-carbon double bond being non-conjugated to the first one.Preferred dienes are selected from C₈ to C₁₄ non-conjugated dienes ormixtures thereof, more preferably selected from 1,7-octadiene,1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene,7-methyl-1,6-octadiene, 9-methyl-1,8-decadiene, or mixtures thereof.Even more preferably, the diene is selected from 1,7-octadiene,1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, or any mixturethereof, however, without limiting to above dienes.

It is well known that e.g. propylene can be used as a comonomer or as achain transfer agent (CTA), or both, whereby it can contribute to thetotal amount of the C—C double bonds, preferably to the total amount ofthe vinyl groups. Herein, when a compound which can also act ascomonomer, such as propylene, is used as CTA for providing double bonds,then said copolymerisable comonomer is not calculated to the comonomercontent.

If the polyolefin, more preferably the LDPE polymer, is unsaturated,then it has preferably a total amount of carbon-carbon double bonds,which originate from vinyl groups, vinylidene groups and trans-vinylenegroups, if present, of more than 0.5/1000 carbon atoms. The upper limitof the amount of carbon-carbon double bonds present in the polyolefin isnot limited and may preferably be less than 5.0/1000 carbon atoms,preferably less than 3.0/1000 carbon atoms.

In some embodiments, wherein e.g. higher crosslinking level of the finalcrosslinked insulation layer is desired, the total amount ofcarbon-carbon double bonds, which originate from vinyl groups,vinylidene groups and trans-vinylene groups, if present, in theunsaturated LDPE, is preferably higher than 0.50/1000 carbon atoms,preferably higher than 0.60/1000 carbon atoms.

If desired the higher double bond content combined with the presence ofthe crosslinking agent, preferably peroxide, provides to the Polymercomposition an advantageous balance between electrical, and mechanicalproperties, preferably combined with good heat and deformationresistance.

Accordingly, the polyolefin is preferably unsaturated and contains atleast vinyl groups and the total amount of vinyl groups is preferablyhigher than 0.05/1000 carbon atoms, still more preferably higher than0.08/1000 carbon atoms, and most preferably of higher than 0.11/1000carbon atoms. Preferably, the total amount of vinyl groups is of lowerthan 4.0/1000 carbon atoms. More preferably, the polyolefin, prior tocrosslinking, contains vinyl groups in total amount of more than0.20/1000 carbon atoms, still more preferably of more than 0.30/1000carbon atoms, and most preferably of more than 0.40/1000 carbon atoms.In some demanding embodiments, preferably in power cables, morepreferably in DC power cables, at least one layer, preferably theinsulation layer, comprises LDPE polymer, preferably LDPE copolymer,which contains vinyl groups in total amount of more than 0.50/1000carbon atoms.

The preferred polyolefin for use in the Polymer composition is asaturated LDPE homopolymer or a saturated LDPE copolymer of ethylenewith one or more comonomer(s); or an unsaturated LDPE polymer, which isselected from an unsaturated LDPE homopolymer or an unsaturated LDPEcopolymer of ethylene with one or more comonomer(s), even morepreferably an unsaturated LDPE homopolymer or an unsaturated LDPEcopolymer of ethylene with one or more comonomer(s), which is preferablyat least one polyunsaturated comonomer, preferably a diene as definedabove, and optionally with other comonomer(s), and has the total amountof carbon-carbon double bonds, which originate from vinyl groups,vinylidene groups and trans-vinylene groups, if present, as definedabove, preferably has the total amount of vinyl groups as defined above.Said unsaturated LDPE polymer is highly usable for an insulation layerof a power cable, preferably of a DC power cable, of the invention.

Typically, and preferably in W&C applications, the density of thepolyolefin, preferably of the LDPE polymer, is higher than 860 kg/m³.Preferably the density of the polyolefin, preferably of the LDPEpolymer, the ethylene homo- or copolymer is not higher than 960 kg/m³,and preferably is from 900 to 945 kg/m³. The MFR2 (2.16 kg, 190° C.) ofthe polyolefin, preferably of the LDPE polymer, is preferably from 0.01to 50 g/10min, more preferably is from 0.1 to 20 g/10min, and mostpreferably is from 0.2 to 10 g/10min.

Compressor Lubricant

The compressor lubricant used in the polymerization process forproducing the preferred polyolefin of the Polymer composition comprisesmineral oil which is a known petroleum product.

Mineral oils have a well known meaning and are used i.a. for lubricationin commercial lubricants. “Compressor lubricant comprising a mineraloil” and “mineral oil-based compressor lubricants” are used hereininterchangeably.

Mineral oil can be a synthetic mineral oil which is producedsynthetically or a mineral oil obtainable from crude oil refineryprocesses.

Typically, mineral oil, known also as liquid petroleum, is a by-productin the distillation of petroleum to produce gasoline and other petroleumbased products from crude oil.

The mineral oil of the compressor lubricant of the invention ispreferably a paraffinic oil. Such paraffinic oil is derived frompetroleum based hydrocarbon feedstocks.

Mineral oil is preferably the base oil of the compressor lubricant. Thecompressor lubricant may comprise other components, such as lubricityadditive(s), viscosity builders, antioxidants, other additive(s) or anymixtures thereof, as well known in the art.

More preferably, the compressor lubricant comprises a mineral oil whichis conventionally used as compressor lubricants for producing plastics,e.g. LDPE, for food or medical industry, more preferably the compressorlubricant comprises a mineral oil which is a white oil. Even morepreferably the compressor lubricant comprises white oil as the mineraloil and is suitable for the production of polymers for food or medicalindustry. White oil has a well known meaning. Moreover such white oilbased compressor lubricants are well known and commercially available.Even more preferably the white oil meets the requirements for a food ormedical white oil.

As, known, the mineral oil, preferably the white mineral oil of thepreferred compressor lubricant contains paraffinic hydrocarbons.

Even more preferably, of the compressor lubricant meets one or more ofthe below embodiments:

In one preferable embodiment, the mineral oil, preferably the whitemineral oil, of the compressor lubricant has a viscosity of at least8.5×10⁻⁶ m²/s at 100° C.;

In a second preferable embodiment, the mineral oil, preferably the whitemineral oil, of the compressor lubricant contains 5% per weight (wt %)or less of hydrocarbons with less than 25 carbon atoms;

In a third preferable embodiment, the hydrocarbons of the mineral oil,preferably of the white mineral oil, of the compressor lubricant have anaverage molecular weight (Mw) of 480 or more.

The above “amount of hydrocarbons”, “viscosity” and “Mw” are preferablyin accordance with the above European Directive 2002/72/EC of 6 Aug.2002.

It is preferred that the compressor lubricant is according to each ofthe above three embodiments 1-3.

The most preferred compressor lubricant of the invention meets therequirements given for white mineral oil in European Directive2002/72/EC of 6 Aug. 2002, Annex V, for plastics used in food contact.Directive is published e.g., in L 220/18 EN Official Journal of theEuropean Communities 15 Aug. 2002. Accordingly the mineral oil is mostpreferably a white mineral oil which meets said European Directive2002/72/EC of 6 August 2002, Annex V. Moreover it is preferred that thecompressor lubricant complies with said European Directive 2002/72/EC of6 Aug. 2002.

The compressor lubricant of the invention can be a commerciallyavailable compressor lubricant or can be produced by conventional means,and is preferably a commercial lubricant used in high pressurepolymerisation processes for producing plastics for medical or foodapplications. Non-exhaustive examples of preferable commerciallyavailable compressor lubricants are e.g. Exxcolub R Series compressorlubricant for production of polyethylene used in food contact andsupplied i.a. by ExxonMobil, Shell Corena for producing polyethylene forpharmaceutical use and supplied by Shell, or CL-1000-SONO-EU, suppliedby Sonneborn.

The compressor lubricant contains preferably no polyalkyleneglycol basedcomponents.

It is preferred that any mineral oil present in the Polymer compositionof the invention originates from the compressor lubricant used in theprocess equipment during the polymerisation process of the polyolefin.Accordingly, it is preferred that no mineral oil is added to the Polymercomposition or to the polyolefin after the polymerisation thereof.

Traces of the mineral oil originating from the compressor lubricant andpresent, if any, in the produced polyolefin would typically amount inmaximum of up to 0.4 wt % based on the amount of the polyolefin. Thegiven limit is the absolute maximum based on the calculation of theworst scenario where all the lost compressor lubricant (average leakage)would go to the final polyolefin. Such worst scenario is unlikely andnormally the resulting polyolefin contains clearly lower level of themineral oil.

The compressor lubricant of the invention is used in a conventionalmanner and well known to a skilled person for the lubrication of thecompressor(s) in the compressing step (a) of the invention.

Process

The high pressure (HP) process is the preferred process for producing apolyolefin of the Polymer composition, preferably a low densitypolyethylene (LDPE) polymer selected from LDPE homopolymer or LDPEcopolymer of ethylene with one or more comonomers.

The invention further provides a process for polymerising a polyolefinin a high pressure process which comprises the steps of:

(a) compressing one or more monomer(s) under pressure in a compressor,wherein a compressor lubricant is used for lubrication,

(b) polymerising a monomer optionally together with one or morecomonomer(s) in a polymerisation zone(s),

(c) separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone,

wherein in step a) a compressor lubricant comprises a mineral oilincluding the preferable embodiments thereof.

Accordingly, the polyolefin of the invention is preferably produced athigh pressure by free radical initiated polymerisation (referred to ashigh pressure radical polymerization). The preferred polyolefin is LDPEhomopolymer or LDPE copolymer of ethylene with one or more comonomer(s),as defined above. The LDPE polymer obtainable by the process of theinvention preferably provides the advantageous electrical properties asdefined above or below. The high pressure (HP) polymerisation and theadjustment of process conditions for further tailoring the otherproperties of the polyolefin depending on the desired end applicationare well known and described in the literature, and can readily be usedby a skilled person.

Compression Step a) of the Process of the Invention:

Monomer, preferably ethylene, with one or more optional comonomer(s), isfed to one or more compressor at compressor zone to compress themonomer(s) up to the desired polymerization pressure and to enablehandling of high amounts of monomer(s) at controlled temperature.Typical compressors, i.e. hyper-compressors, for the process can bepiston compressors or diaphragm compressors. The compressor zone usuallycomprises one or more compressor(s), i.e. hyper-compressor(s), which canwork in series or in parallel. The compressor lubricant of the inventionis used for cylinder lubrication in at least one, preferably in all ofthe hyper-compressor(s), present in the compressor zone. The compressionstep a) comprises usually 2-7 compression steps, often with intermediatecooling zones. Temperature is typically low, usually in the range ofless than 200° C., preferably of less than 100° C. Any recycled monomer,preferably ethylene, and optional comonomer(s) can be added at feasiblepoints depending on the pressure.

Polymerisation Step b) of the Process:

Preferred high pressure polymerisation is effected at a polymerisationzone which comprises one or more polymerisation reactor(s), preferablyat least a tubular reactor or an autoclave reactor, preferably a tubularreactor. The polymerization reactor(s), preferably a tubular reactor,may comprise one or more reactor zones, wherein different polymerizationconditions may occur and/or adjusted as well known in the HP field. Oneor more reactor zone(s) are provided in a known manner with means forfeeding monomer and optional comonomer(s), as well as with means foradding initiator(s) and/or further components, such as CTA(s).Additionally, the polymerization zone may comprise a preheating sectionwhich is preceding or integrated to the polymerization reactor. In onepreferable HP process the monomer, preferably ethylene, optionallytogether with one or more comonomer(s) is polymerized in a preferabletubular reactor, preferably in the presence of chain transfer agent(s).

Tubular Reactor:

The reaction mixture is fed to the tubular reactor. The tubular reactormay be operated as a single-feed system (also known as front feed),wherein the total monomer flow from the compressor zone is fed to theinlet of the first reaction zone of the reactor. Alternatively thetubular reactor may be a multifeed system, wherein e.g the monomer(s),the optional comonomer(s) or further component(s) (like CTA(s)) comingfrom the compression zone, separately or in any combinations, is/aresplit to two or more streams and the split feed(s) is introduced to thetubular reactor to the different reaction zones along the reactor. Forinstance 10-90% of the total monomer quantity is fed to the firstreaction zone and the other 90-10% of the remaining monomer quantity isoptionally further split and each split is injected at differentlocations along the reactor. Also the feed of initiator(s) may be splitin two or more streams. Moreover, in a multifeed system the splitstreams of monomer(/comonomer) and/or optional further component(s),such as CTA, and, respectively, the split streams of initiator(s) mayhave the same or different component(s) or concentrations of thecomponents, or both.

The single feed system for the monomer and optional comonomer(s) ispreferred in the tubular reactor for producing the polyolefin of theinvention.

First part of the tubular reactor is to adjust the temperature of thefeed of monomer, preferably ethylene, and the optional comonomer(s);usual temperature is below 200° C., such as 100-200° C. Then the radicalinitiator is added. As the radical initiator, any compound or a mixturethereof that decomposes to radicals at an elevated temperature can beused. Usable radical initiators, such as peroxides, are commerciallyavailable. The polymerization reaction is exothermic. There can beseveral radical initiator injections points, e.g. 1-5 points, along thereactor usually provided with separate injection pumps. As alreadymentioned also the monomer, preferably ethylene, and optionalcomonomer(s), is added at front and optionally the monomer feed(s) canbe split for the addition of the monomer and/or optional comonomer(s),at any time of the process, at any zone of the tubular reactor and fromone or more injection point(s), e.g. 1-5 point(s), with or withoutseparate compressors.

Furthermore, one or more CTA(s) are preferably used in thepolymerization process of the Polyolefin. Preferred CTA(s) can beselected from one or more non-polar and one or more polar CTA(s), or anymixtures thereof.

Non-polar CTA, if present, is preferably selected from

i) one or more compound(s) which does not contain a polar group selectedfrom nitrile (CN), sulfide, hydroxyl, alkoxy, aldehyl (HC═O), carbonyl,carboxyl, ether or ester group(s), or mixtures thereof. Non-polar CTA ispreferably selected from one or more non-aromatic, straight chainbranched or cyclic hydrocarbyl(s), optionally containing a hetero atomsuch as O, N, S, Si or P. More preferably the non-polar CTA(s) isselected from one or more cyclic alpha-olefin(s) of 5 to 12 carbon orone or more straight or branched chain alpha-olefin(s) of 3 to 12 carbonatoms, more preferably from one or more straight or branched chainalpha-olefin(s) of 3 to 6 carbon atoms. The preferred non-polar CTA ispropylene.

The polar CTA, if present, is preferably selected from

i) one or more compound(s) comprising one or more polar group(s)selected from nitrile (CN), sulfide, hydroxyl, alkoxy, aldehyl (HC═O),carbonyl, carboxyl, ether or ester group(s), or mixtures thereof;

ii) one or more aromatic organic compound(s), or

iii) any mixture thereof.

Preferably any such polar CTA(s) have up to 12 carbon atoms, e.g. up to10 carbon atoms preferably up to 8 carbon atoms. A preferred optionincludes a straight chain or branched chain alkane(s) having up to 12carbon atoms (e.g. up to 8 carbon atoms) and having at least one nitrile(CN), sulfide, hydroxyl, alkoxy, aldehyl (HC═O), carbonyl, carboxyl orester group.

More preferably the polar CTA(s), if present, is selected from i) one ormore compound(s) containing one or more hydroxyl, alkoxy, HC═O,carbonyl, carboxyl and ester group(s), or a mixture thereof, morepreferably from one or more alcohol, aldehyde and/or ketone compound(s).The preferred polar CTA(s), if present, is a straight chain or branchedchain alcohol(s), aldehyde(s) or ketone(s) having up to 12 carbon atoms,preferably up to 8 carbon atoms, especially up to 6 carbon atoms, mostpreferably, isopropanol (IPA), methylethylketone (MEK) and/orpropionaldehyde (PA).

The amount of the preferable CTA(s) is not limited and can be tailoredby a skilled person within the limits of the invention depending on thedesired end properties of the final polymer. Accordingly, the preferablechain transfer agent(s) can be added in any injection point of thereactor to the polymer mixture. The addition of one or more CTA(s) canbe effected from one or more injection point(s) at any time during thepolymerization.

In case the polymerization of the polyolefin is carried out in thepresence of a CTA mixture comprising one or more polar CTA(s) as definedabove and one or more non-polar CTA(s) as defined above, then the feedratio by weight% of polar CTA to non-polar CTA is preferably

1 to 99 wt % of polar CTA and

1 to 99wt % of non-polar CTA, based on the combined amount of the feedof polar CTA and the non-polar CTA into the reactor.

The addition of monomer, comonomer(s) and optional CTA(s) may includeand typically includes fresh and recycled feed(s).

The reactor is continuously cooled e.g. by water or steam. The highesttemperature is called peak temperature and the reaction startingtemperature is called initiation temperature.

Suitable temperatures range up to 400° C., preferably from 80 to 350° C.and pressure from 700 bar, preferably 1000 to 4000 bar, more preferablyfrom 1000 to 3500 bar. Pressure can be measured at least aftercompression stage and/or after the tubular reactor. Temperature can bemeasured at several points during all steps. High temperature and highpressure generally increase output. Using various temperature profilesselected by a person skilled in the art will allow control of structureof polymer chain, i.e. Long Chain Branching and/or Short Chainbranching, density, branching factor, distribution of comonomers, MFR,viscosity, Molecular Weight Distribution etc.

The reactor ends conventionally with a valve a so-called productioncontrol valve. The valve regulates reactor pressure and depressurizesthe reaction mixture from reaction pressure to separation pressure.

Recovering step c) of the process:

Separation:

The pressure is typically reduced to approx 100 to 450 bar and thereaction mixture is fed to a separator vessel where most of theunreacted, often gaseous, products are removed from the polymer stream.Unreacted products comprise e.g. monomer or the optional comonomer(s),and most of the unreacted components are recovered. The polymer streamis optionally further separated at lower pressure, typically less than 1bar, in a second separator vessel where more of the unreacted productsare recovered. Normally low molecular compounds, i.e. wax, are removedfrom the gas. The gas is usually cooled and cleaned before recycling.

Recovery of the separated polymer:

After the separation the obtained polymer is typically in a form of apolymer melt which is normally mixed and pelletized in a pelletisingsection, such as pelletising extruder, arranged in connection to the HPreactor system. Optionally, additive(s), such as antioxidant(s), can beadded in this mixer in a known manner to result in the Polymercomposition.

Further details of the production of ethylene (co)polymers by highpressure radical polymerization can be found i.a. in the Encyclopedia ofPolymer Science and Engineering, Vol. 6 (1986), pp 383-410 andEncyclopedia of Materials: Science and Technology, 2001 Elsevier ScienceLtd.: “Polyethylene: High-pressure, R. Klimesch, D. Littmann and F.-O.Mähling pp. 7181-7184.

As to polymer properties, e.g. MFR, of the polymerised Polymer,preferably LDPE polymer, the properties can be adjusted by using e.g.chain transfer agent during the polymerisation, or by adjusting reactiontemperature or pressure (which also to a certain extent have aninfluence on the unsaturation level).

When an unsaturated LDPE copolymer of ethylene is prepared, then, aswell known, the C—C double bond content can be adjusted by polymerisingthe ethylene e.g. in the presence of one or more polyunsaturatedcomonomer(s), chain transfer agent(s), process conditions, or anycombinations thereof, e.g. using the desired feed ratio between monomer,preferably ethylene, and polyunsaturated comonomer and/or chain transferagent, depending on the nature and amount of C—C double bonds desiredfor the unsaturated LDPE copolymer. I.a. WO 9308222 describes a highpressure radical polymerisation of ethylene with polyunsaturatedmonomers, such as an α,ω-alkadienes, to increase the unsaturation of anethylene copolymer. The non-reacted double bond(s) thus provides i.a.pendant vinyl groups to the formed polymer chain at the site, where thepolyunsaturated comonomer was incorporated by polymerization. As aresult the unsaturation can be uniformly distributed along the polymerchain in random copolymerisation manner. Also e.g. WO 9635732 describeshigh pressure radical polymerisation of ethylene and a certain type ofpolyunsaturated α,ω-divinylsiloxanes. Moreover, as known, e.g. propylenecan be used as a chain transfer agent to provide said double bonds.

Polymer Composition

Prior to crosslinking the polymer composition comprises at least onecrosslinking agent, preferably at least one peroxide which contains atleast one —O—O— bond. Naturally, in case of two or more differentperoxide products are used in the polymer composition, then thepreferred amount (in mmol) of —O—O-/kg polymer composition as definedabove, below or in claims is the sum of the amount of —O—O-/kg polymercomposition of each peroxide product.

The Polymer composition of the invention comprises typically at least 50wt %, preferably at least 60 wt %, more preferably at least 70wt %, morepreferably at least 75 wt %, more preferably from 80 to 100 wt % andmore preferably from 85 to 100 wt %, of the polyolefin based on thetotal weight of the polymer component(s) present in the Polymercomposition. The preferred Polymer composition consists of polyolefin asthe only polymer component. The expression means that the Polymercomposition does not contain further polymer components, but thepolyolefin as the sole polymer component. However, it is to beunderstood herein that the Polymer composition may comprise furthercomponent(s) other than polymer components, such as additive(s) whichmay optionally be added in a mixture with a carrier polymer, i.e. in socalled master batch.

The Polymer composition comprises preferably conventionally usedadditive(s) for W&C applications, such as one or more antioxidant(s) andoptionally one or more scorch retarder(s), preferably at least one ormore antioxidant(s). The used amounts of additives are conventional andwell known to a skilled person, e.g. as already described above under“Description of the invention”.

The Polymer composition preferably consist of the Polyolefin, which ispreferably polyethylene, more preferably LDPE homo or copolymer whichmay optionally be unsaturated, of the invention as the sole polymercomponent. The most preferred polyolefin of the Polymer composition isan unsaturated LDPE homo or copolymer.

End Uses and End Applications of the Invention

As mentioned above, the new Polymer composition is highly useful in widevariety of W&C applications, more preferably in one or more layers of apower cable.

A power cable is defined to be a cable transferring energy operating atany voltage, typically operating at voltages higher than 1 kV. Thevoltage applied to the power cable can be alternating (AC), direct (DC),or transient (impulse). The polymer composition of the invention is verysuitable for power cables, especially for power cables operating atvoltages higher than 6 kV to 36 kV (medium voltage (MV) cables) and atvoltages higher than 36 kV, known as high voltage (HV) cables and extrahigh voltage (EHV) cables, which EHV cables operate, as well known, atvery high voltages. The terms have well known meanings and indicate theoperating level of such cables. For HV and EHV DC power cables theoperating voltage is defined herein as the electric voltage betweenground and the conductor of the high voltage cable. HV DC power cableand EHV DC power cable can operate e.g. at voltages of 40 kV or higher,even at voltages of 50 kV or higher. EHV DC power cables operate at veryhigh voltage ranges e.g as high as up to 800 kV, however withoutlimiting thereto.

The Polymer composition with advantageous DC conductivity properties isalso highly suitable for direct current (DC) power cables operating atany voltages, preferably at higher than 36 kV, such as HV or EHV DCpower cables, as defined above.

In addition to reduced electrical conductivity, the Polymer compositionhas preferably also very good space charge properties which areadvantageous for power cables, particularly for DC power cables.

The invention further provides the use of the polyolefin of theinvention, which is obtainable by the high pressure (HP) process of theinvention, for producing an insulation layer of a power cable,preferably of a DC power cable.

A crosslinkable power cable, preferably a crosslinkable DC power cable,is provided comprising a conductor surrounded by one or more layers,preferably at least an insulation layer, more preferably at least aninner semiconductive layer, an insulation layer and an outersemiconductive layer, in that order, wherein at least one of saidlayer(s), preferably the insulation layer, comprises a polymercomposition of the invention comprising a crosslinkable polyolefin and acrosslinking agent, preferably peroxide, which is preferably in anamount of at least 35 mmol —O—O-/kg polymer composition, preferably ofat least 36 mmol —O—O-/kg polymer composition, 37 mmol —O—O-/kg polymercomposition or more, preferably from 37 to 90 mmol —O—O-/kg polymercomposition, more preferably 37 to 75 mmol —O—O-/kg polymer composition.The insulation layer of the power cable, preferably of the DC powercable, preferably comprises said crosslinkable unsaturated LDPEcopolymer as defined above.

The term “conductor” means herein above and below that the conductorcomprises one or more wires. Moreover, the cable may comprise one ormore such conductors. Preferably the conductor is an electricalconductor and comprises one or more metal wires.

The invention also provides a process for producing a power cable,preferably a crosslinkable power cable, more preferably a crosslinkableDC power cable, more preferably a crosslinkable HV or EHV DC powercable, as defined above or in claims comprising a conductor surroundedby one or more layers, preferably at least an insulation layer, morepreferably at least an inner semiconductive layer, an insulation layerand an outer semiconductive layer, in that order, wherein the processcomprises the steps of applying one or more layers on a conductorwherein at least one layer, preferably the insulation layer, comprises acrosslinkable polymer composition of the invention comprising apolyolefin and a crosslinking agent, preferably peroxide, preferably inan amount of at least 35 mmol —O—O-/kg polymer composition, preferablyof at least 36 mmol —O—O-/kg polymer composition, 37 mmol —O—O-/kgpolymer composition or more, preferably from 37 to 90 mmol —O—O-/kgpolymer composition, more preferably 37 to 75 mmol —O—O-/kg polymercomposition.

In the preferred embodiment of the power cable production process of theinvention a crosslinkable power cable is produced by

(a) providing and mixing, preferably meltmixing in an extruder, saidcrosslinkable polymer composition of the invention as defined above orbelow in claims,

(b) applying a meltmix of the polymer composition obtained from step(a), preferably by (co)extrusion, on a conductor to form one or morelayers, preferably at least an insulation layer, and

(c) optionally crosslinking at least the polymer composition in said atleast one layer, preferably in the insulation layer.

More preferably a crosslinkable DC power cable, preferably acrosslinkable HV or EHV DC power cable, is produced comprising aconductor surrounded by an inner semiconductive layer, an insulationlayer, and an outer semiconductive layer, in that order, wherein theprocess comprises the steps of

(a)

providing and mixing, preferably meltmixing in an extruder, acrosslinkable first semiconductive composition comprising a polymer, acarbon black and optionally further component(s) for the innersemiconductive layer,

providing and mixing, preferably meltmixing in an extruder, acrosslinkable polymer composition of the invention for the insulationlayer,

providing and mixing, preferably meltmixing in an extruder, a secondsemiconductive composition which is preferably crosslinkable andcomprises a polymer, a carbon black and optionally further component(s)for the outer semiconductive layer,

(b) applying on a conductor, preferably by coextrusion,

a meltmix of the first semiconductive composition obtained from step (a)to form the inner semiconductive layer,

a meltmix of polymer composition of the invention obtained from step (a)to form the insulation layer, and

a meltmix of the second semiconductive composition obtained from step(a) to form the outer semiconductive layer, and

(c) optionally crosslinking at crosslinking conditions one or more ofthe polymer composition of the insulation layer, the semiconductivecomposition of the inner semiconductive layer and the semiconductivecomposition of the outer semiconductive layer, of the obtained cable,preferably at least the polymer composition of the insulation layer,more preferably the polymer composition of the insulation layer and atleast one, preferably both, of the semiconductive composition of theinner semiconductive layer and the semiconductive composition of theouter semiconductive layer.

The polymer of the first and the second semiconductive composition ispreferably selected independently from a polyolefin, e.g. as describedin relation to the polymer composition of the invention. The carbonblack can be any conventional carbon black used in the semiconductivelayers of a power cable, preferably in the semiconductive layer of a DCpower cable. Examples of carbon blacks are conductive carbon black, suchas the well known furnace carbon black and acetylene carbon black.Moreover the first and second semiconductive compositions may bedifferent or identical, preferably identical.

Melt mixing means mixing above the melting point of at least the majorpolymer component(s) of the obtained mixture and is typically carriedout in a temperature of at least 10-15° C. above the melting orsoftening point of polymer component(s).

The term “(co)extrusion” means herein that in case of two or morelayers, said layers can be extruded in separate steps, or at least twoor all of said layers can be coextruded in a same extrusion step, aswell known in the art. The term “(co)extrusion” means herein also thatall or part of the layer(s) are formed simultaneously using one or moreextrusion heads. For instance triple extrusion can be used for formingthree cable layers.

As well known, the polymer composition of the invention and the optionaland preferred first and second semiconductive compositions can beproduced before or during the cable production process. Moreover thepolymer composition of the invention and the optional and preferredfirst and second semiconductive composition can each independentlycomprise part or all of the component(s) thereof before introducing tothe (melt)mixing step a) of the cable production process.

Preferably, said part or all of the Polymer composition, preferably atleast the Polyolefin, is in form of powder, grain or pellets, whenprovided to the cable production process. Pellets can be of any size andshape and can be produced by any conventional pelletising device, suchas a pelletising extruder.

According to one embodiment, the Polymer composition comprises saidoptional further component(s). In this embodiment part or all of saidfurther component(s) may e.g. be added

1) by meltmixing to the Polyolefin, which may be in a form as obtainedfrom a polymerisation process, and then the obtained meltmix ispelletised, and/or

2) by mixing to the pellets of the Polyolefin which pellets may alreadycontain part of said further component(s). In this option 2) part or allof the further component(s) can be meltmixed together with the pelletsand then the obtained meltmix is pelletised; and/or part or all of thefurther components can be impregnated to the solid pellets.

In alternative second embodiment, the Polymer composition may beprepared in connection with the cable production line e.g. by providingthe Polyolefin, preferably in form of pellets which may optionallycomprise part of the further component(s), and combined with all or restof the further component(s) in the mixing step a) to provide a (melt)mixfor the step b) of the process of the invention. In case the pellets ofthe Polyolefin contain part of the further component(s), then thepellets may be prepared as described in the above first embodiment.

The further component(s) is preferably selected at least from one ormore additive(s), preferably at least from free radical generatingagent(s), more preferably from peroxide(s), optionally, and preferably,from antioxidant(s) and optionally from scorch retardant(s) as mentionedabove.

The mixing step a) of the provided Polymer composition and preferablefirst and second semiconductive compositions is preferably carried outin the cable extruder. The step a) may optionally comprise a separatemixing step, e.g. in a mixer, preceding the cable extruder. Mixing inthe preceding separate mixer can be carried out by mixing with orwithout external heating (heating with an external source) of thecomponent(s). Any further component(s) of the Polymer composition or thepreferable first and second semiconductive composition, if present andadded during the cable production process, can be added at any stage andany point(s) in to the cable extruder, or to the optional separate mixerpreceding the cable extruder. The addition of additives can be madesimultaneously or separately as such, preferably in liquid form, or in awell known master batch, and at any stage during the mixing step (a).

It is preferred that the (melt)mix of the Polymer composition obtainedfrom (melt)mixing step (a) consists of the polyolefin of the inventionas the sole polymer component. The optional, and preferable, additive(s)can be added to Polymer composition as such or as a mixture with acarrier polymer, i.e. in a form of so-called master batch.

Most preferably the mixture of the polymer composition of the inventionand the mixture of each of the optional first and second semiconductivecomposition obtained from step (a) is a meltmix produced at least in anextruder.

In the preferred embodiment the polymer composition of the invention isprovided to the cable production process in a form of premade pellets.

In a preferred embodiment of the cable production process, acrosslinkable power cable, preferably a crosslinkable DC power cable,more preferably a crosslinkable HV or EHV DC power cable, is produced,wherein the insulation layer comprises the polymer composition of theinvention comprising a crosslinkable polyolefin, preferably anunsaturated LDPE homo or copolymer, and a crosslinking agent, preferablyperoxide, in an amount as given above or below, and then thecrosslinkable polyolefin in the insulation layer of the obtained cableis crosslinked in step c) in crosslinking conditions. More preferably inthis embodiment, a crosslinked power cable, preferably a crosslinked DCpower cable, more preferably a crosslinked HV or EHV DC power cable, isproduced, which comprises a conductor surrounded by an innersemiconductive layer comprising, preferably consisting of, a firstsemiconductive composition, an insulation layer comprising, preferablyconsisting of, a crosslinkable polymer composition of the inventioncomprising a polyolefin and a crosslinking agent, preferably peroxide,as defined above, and optionally, and preferably, an outersemiconductive layer comprising, preferably consisting of, a secondsemiconductive composition,

wherein the polymer composition of the insulation layer is crosslinkedat crosslinking conditions in step (c) in the presence of a crosslinkingagent, preferably peroxide, preferably in an amounts as defined above orclaims, and wherein the first semiconductive composition of the innersemiconductive, and optionally, and preferably, the secondsemiconductive composition of the outer semiconductive layer arecrosslinked at crosslinking conditions in step (c) in the presence ofcrosslinking agent(s), preferably in the presence of free radicalgenerating agent(s), which is preferably a peroxide(s).

The crosslinking agent(s) can already be present in the optional firstand second semiconductive composition before introducing to thecrosslinking step (c) or introduced during the crosslinking step (c).Peroxide is the preferred crosslinking agent for said optional first andsecond semiconductive compositions and is preferably included to thepellets of semiconductive composition before the composition is used inthe cable production process as described above.

Crosslinking can be carried out at increased temperature which ischosen, as well known, depending on the type of crosslinking agent. Forinstance temperatures above 150° C. are typical, however withoutlimiting thereto.

The invention further provides a crosslinked power cable, preferably acrosslinked DC power cable, comprising a conductor surrounded by one ormore layers, preferably at least by an insulation layer, more preferablyat least by an inner semiconductive layer, insulation layer and an outersemiconductive layer, in that order, wherein at least the insulationlayer comprises the crosslinked polymer composition or any of thepreferable subgroups or embodiments thereof as defined above or inclaims. More preferably also the inner semiconductive composition andthe optional and preferred outer semiconductive composition arecrosslinked.

Naturally, the polymer composition of the invention used in at least onecable layer, preferably in an insulation layer, of the cable of theinvention has, when crosslinked, the advantageous electrical propertiesas defined above or in claims.

The use of the Polymer composition, or any of the preferable subgroupsor embodiments thereof, as defined above or in claims in at least one oflayer, preferably in at least an insulation layer, of a crosslinkedpower cable, preferably of a crosslinked (DC) power cable, preferably ofa crosslinked HV or EHV DC power cable, comprising a conductorsurrounded by at least one layer, preferably at least an innersemiconductive layer, insulation layer and an outer semiconductivelayer, in that order, and optionally surrounded by one or more otherlayer(s), such as screen(s), a jacketing layer(s) or other protectivelayer(s), as well known in the art.

The thickness of the insulation layer of the power cable, preferably ofthe DC cable, more preferably of the HV or EHV DC power cable, istypically 2 mm or more, preferably at least 3 mm, preferably of at least3 to 100 mm, when measured from a cross section of the insulation layerof the cable.

The invention further provides a use of the polymer composition of theinvention in an insulation layer of a crosslinked (DC) power cable,preferably of a crosslinked HV or EHV DC power cable, comprising aconductor surrounded by at least one layer, preferably at least an innersemiconductive layer, insulation layer and an outer semiconductivelayer, in that order, for reducing the DC conductivity of said cable.

Determination Methods

Unless otherwise stated in the description or experimental part thefollowing methods were used for the property determinations.

Wt %: % by weight

Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1133 and isindicated in g/10 min. The MFR is an indication of the flowability, andhence the processability, of the polymer. The higher the melt flow rate,the lower the viscosity of the polymer. The MFR is determined at 190° C.for polyethylenes and may be determined at different loadings such as2.16 kg (MFR2) or 21.6 kg (MFR21).

Density

The density was measured according to ISO 1183-2. The sample preparationwas executed according to ISO 1872-2 Table 3 Q (compression moulding).

Molecular Weight

The Mz, Mw, Mn, and MWD are measured by Gel Permeation Chromatography(GPC) for low molecular weight polymers as known in the field.

Comonomer Contents

a) Quantification of Alpha-Olefin content in Linear Low DensityPolyethylenes and Low Density Polyethylenes by NMR Spectroscopy:

The comonomer content was determined by quantitative 13C nuclearmagnetic resonance (NMR) spectroscopy after basic assignment (J. RandallJ M S—Rev. Macromol. Chem. Phys., C29(2&3), 201-317 (1989)).Experimental parameters were adjusted to ensure measurement ofquantitative spectra for this specific task.

Specifically solution-state NMR spectroscopy was employed using a BrukerAvanceIII 400 spectrometer. Homogeneous samples were prepared bydissolving approximately 0.200 g of polymer in 2.5 ml ofdeuterated-tetrachloroethene in 10 mm sample tubes utilising a heatblock and rotating tube oven at 140° C. Proton decoupled 13C singlepulse NMR spectra with NOE (powergated) were recorded using thefollowing acquisition parameters: a flip-angle of 90 degrees, 4 dummyscans, 4096 transients an acquisition time of 1.6 s, a spectral width of20 kHz, a temperature of 125° C., a bilevel WALTZ proton decouplingscheme and a relaxation delay of 3.0 s. The resulting FID was processedusing the following processing parameters: zero-filling to 32 k datapoints and apodisation using a gaussian window function; automaticzeroth and first order phase correction and automatic baselinecorrection using a fifth order polynomial restricted to the region ofinterest.

Quantities were calculated using simple corrected ratios of the signalintegrals of representative sites based upon methods well known in theart.

b) Comonomer Content of Polar Comonomers in Low Density Polyethylene

(1) Polymers Containing >6 wt.% Polar Comonomer Units

Comonomer content (wt %) was determined in a known manner based onFourier transform infrared spectroscopy (FTIR) determination calibratedwith quantitative nuclear magnetic resonance (NMR) spectroscopy. Belowis exemplified the determination of the polar comonomer content ofethylene ethyl acrylate, ethylene butyl acrylate and ethylene methylacrylate. Film samples of the polymers were prepared for the FTIRmeasurement: 0.5-0.7 mm thickness was used for ethylene butyl acrylateand ethylene ethyl acrylate and 0.10 mm film thickness for ethylenemethyl acrylate in amount of >6 wt %. Films were pressed using a Specacfilm press at 150° C., approximately at 5 tons, 1-2 minutes, and thencooled with cold water in a not controlled manner. The accuratethickness of the obtained film samples was measured.

After the analysis with FTIR, base lines in absorbance mode were drawnfor the peaks to be analysed. The absorbance peak for the comonomer wasnormalised with the absorbance peak of polyethylene (e.g. the peakheight for butyl acrylate or ethyl acrylate at 3450 cm⁻¹ was dividedwith the peak height of polyethylene at 2020 cm⁻¹). The NMR spectroscopycalibration procedure was undertaken in the conventional manner which iswell documented in the literature, explained below.

For the determination of the content of methyl acrylate a 0.10 mm thickfilm sample was prepared. After the analysis the maximum absorbance forthe peak for the methylacrylate at 3455 cm⁻¹ was subtracted with theabsorbance value for the base line at 2475 cm⁻¹(A_(methylamylate)−A₂₄₇₅). Then the maximum absorbance peak for thepolyethylene peak at 2660 cm⁻¹ was subtracted with the absorbance valuefor the base line at 2475 cm⁻¹ (A₂₆₆₀−A₂₄₇₅). The ratio between(A_(methylacrylate)−A₂₄₇₅) and (A₂₆₆₀−A₂₄₇₅) was then calculated in theconventional manner which is well documented in the literature.

The weight-% can be converted to mol-% by calculation. It is welldocumented in the literature.

Quantification of copolymer content in polymers by NMR spectroscopy

The comonomer content was determined by quantitative nuclear magneticresonance (NMR) spectroscopy after basic assignment (e.g. “NMR Spectraof Polymers and Polymer Additives”, A. J. Brandolini and D. D. Hills,2000, Marcel Dekker, Inc. New York).

Experimental parameters were adjusted to ensure measurement ofquantitative spectra for this specific task (e.g “200 and More NMRExperiments: A Practical Course”, S. Berger and S. Braun, 2004,Wiley-VCH, Weinheim). Quantities were calculated using simple correctedratios of the signal integrals of representative sites in a manner knownin the art.

(2) Polymers Containing 6 wt.% or Less Polar Comonomer Units

Comonomer content (wt. %) was determined in a known manner based onFourier transform infrared spectroscopy (FTIR) determination calibratedwith quantitative nuclear magnetic resonance (NMR) spectroscopy. Belowis exemplified the determination of the polar comonomer content ofethylene butyl acrylate and ethylene methyl acrylate. For the FT-IRmeasurement a film samples of 0.05 to 0.12 mm thickness were prepared asdescribed above under method 1). The accurate thickness of the obtainedfilm samples was measured.

After the analysis with FT-IR base lines in absorbance mode were drawnfor the peaks to be analysed. The maximum absorbance for the peak forthe comonomer (e.g. for methylacrylate at 1164 cm⁻¹ and butylacrylate at1165 cm⁻¹) was subtracted with the absorbance value for the base line at1850 cm⁻¹ (A_(polar comonomer)−A₁₈₅₀). Then the maximum absorbance peakfor polyethylene peak at 2660 cm⁻¹ was subtracted with the absorbancevalue for the base line at 1850 cm⁻¹ (A₂₆₆₀−A₁₈₅₀). The ratio between(A_(comonomer)−-A₁₈₅₀) and (A₂₆₆₀−A₁₈₅₀) was then calculated. The NMRspectroscopy calibration procedure was undertaken in the conventionalmanner which is well documented in the literature, as described aboveunder method 1).

The weight-% can be converted to mol-% by calculation. It is welldocumented in the literature.

Gel Content

The gel content is measured according to ASTM D 2765-01, method B, usingdecaline extraction.

There were however three minor deviations from this standard:

-   -   1) An addition extraction for 1 hour with new decaline was done        in order to secure that all solubles were extracted.    -   2) Only 0.05% antioxidant (Irganox 1076, Octadecyl        3,5-Di-(tert)-butyl-4-hydroxyhydrocinnamate, CAS: 2082-79-3) was        added to the decalin instead of 1% as specified in the standard    -   3) The stripes cut from the cables are 0.2 mm thick, not 0.4 mm        as specified in the standard.

The sample was obtained from the insulation layer, made from the Polymercomposition, as close as possible to the inner semiconductive layer of a20 kV cable. This three layered cable core was produced using a 1+2construction on a pilot scale CCV line. The conductor was made ofAluminium and had an area of 50 mm². The inner semiconductive layer was0.9 mm thick, the insulation layer 5.5 mm thick and the outersemiconductive layer 0.8 mm thick. The line speed used for themanufacturing of the cable cores was 2 m/min. This CCV line has twoheating zones for dry-curing, each of 3 m, and the temperatures used onthese two zones were 450 and 400° C. respectively. The cooling sectionwas 12.8 m long and the cable was cooled with water holding atemperature of around 25-30° C.

DC Conductivity Determination Methods

DC conductivity method 1: Electrical conductivity measured at 70° C. and30 kV/mm mean electric field from a non-degassed or degassed, 1 mmplaque sample consisting of a crosslinked polymer composition.

Plaque Sample Preparation:

The plaques are compression moulded from pellets of the test polymercomposition. The final plaques have a thickness of 1 mm and a diameterof 330 mm.

The plaques are press-moulded at 130° C. for 12 min while the pressureis gradually increased from 2 to 20 MPa. Thereafter the temperature isincreased and reaches 180° C. after 5 min. The temperature is then keptconstant at 180° C. for 15 min during which the plaque becomes fullycrosslinked by means of the peroxide present in the test polymercomposition. Finally the temperature is decreased using the cooling rate15° C./min until room temperature is reached when the pressure isreleased. The plaques are immediately after the pressure release wrappedin metallic foil in order to prevent loss of volatile substances (usedfor the non-degassed determination).

If the plaque is to be degassed it is placed in a vacuum oven atpressure less than 10 Pa and degassed for 24 h at 70° C. Thereafter theplaque is again wrapped in metallic foil in order to prevent furtherexchange of volatile substances between the plaque and the surrounding.

Measurement Procedure:

A high voltage source is connected to the upper electrode, to applyvoltage over the test sample. The resulting current through the sampleis measured with an electrometer. The measurement cell is a threeelectrodes system with brass electrodes. The brass electrodes areequipped with heating pipes connected to a heating circulator, tofacilitate measurements at elevated temperature and provide uniformtemperature of the test sample.

The diameter of the measurement electrode is 100 mm. Silicone rubberskirts are placed between the brass electrode edges and the test sample,to avoid flashovers from the round edges of the electrodes.

The applied voltage was 30 kV DC meaning a mean electric field of 30kV/mm. The temperature was 70° C. The current through the plaque waslogged throughout the whole experiments lasting for 24 hours. Thecurrent after 24 hours was used to calculate the conductivity of theinsulation.

This method and a schematic picture of the measurement setup for theconductivity measurements has been thoroughly described in a publicationpresented at the Nordic Insulation Symposium 2009 (Nord-IS 09),Gothenburg, Sweden, Jun. 15-17, 2009, page 55-58: Olsson et al,“Experimental determination of DC conductivity for XLPE insulation”.

DC conductivity method 2: Electrical conductivity at 20° C. and 40 kV/mmmean electric field from a plaque sample consisting of a crosslinkedpolymer composition

Plaque Sample Preparation:

Pellets of the test polymer composition were compression moulded usingthe following conditions: First the pellets were melted at 120° C. for 1min at 20 bars. Then the temperature was increased to 180° C. at thesame time as the pressure was increased to 200 bars. The plaques thenbecome fully crosslinked by means of the peroxide present in the polymercomposition. The total crosslinking time was 12 min including the timefor increasing the temperature from 120 to 180° C. After completedcrosslinking the plaques were cooled to room temperature with a coolingrate of 15° C./min still under pressure. After removal from the pressthe cooled plaques were degassed in oven at 70° C. for 72 h at 1 atm.The final thickness of the plaques was 0.5 mm.

Conduction Current Measurement:

Conduction current measurement is performed by a three-terminal cell, innitrogen at a pressure of 3 bar and temperature at 20° C. Specimens aretested with gold-coated electrodes obtained by cold sputtering. The lowvoltage electrode has a diameter of 25 mm (measurement area is thus 490mm²). A guard electrode is situated around, but separated from the lowvoltage electrode. The high voltage electrode has a diameter of 50 mm,the same dimension of the external diameter of the guard electrode.

A DC voltage (U) equal to target mean electric stress (E) x measuredinsulation thickness (d) is applied on the high voltage electrode. Thetarget mean electric stress E is in this case 40kV/mm. The currentthrough the tape between the high voltage and the low voltage electrodeis measured with an electrometer. The measurements are terminated whenthe current has reached a steady-state level, normally after 24-48hours. The reported conductivity a is calculated from the steady-statecurrent (I) by the equation

σ=I*d/(A*U)

where A is the measurement area, in this case 490 mm².

DC conductivity method 3: Electrical conductivity measured at 70° C. and30 kV/mm mean electric field from a non-degassed or degassed, 1 mmplaque sample consisting of a crosslinked polymer composition.

The plaques are compression moulded from pellets of the test polymercomposition. The final plaques have a thickness of 1±10% mm and 195×195mm². The thickness is measured at 5 different locations on the plaques.

The plaques are press-moulded at 130° C. for 600 s at 20 Bar. Thereafterthe temperature is increased and reaches 180° C. after 170 s and thepressure is at the same time increased to 200 Bar. The temperature isthen kept constant at 180° C. for 1000 s during which the plaque becomesfully crosslinked by means of the peroxide present in the test polymercomposition. Finally the temperature is decreased using the cooling rate15° C./min until room temperature is reached when the pressure isreleased. The plaque thickness is determined immediately after thecompression moulding and thereafter placed in the test cell describedbelow for conductivity measurement.

A high voltage source is connected to the upper electrode, to applyvoltage over the test sample. The resulting current through the sampleis measured with an electrometer. The measurement cell is a threeelectrodes system with brass electrodes. The cell is installed in aheating oven to facilitate measurements at elevated temperature andprovide uniform temperature of the test sample. The diameter of themeasurement electrode is 100 mm. Silicone rubber skirts are placedbetween the brass electrode edges and the test sample, to avoidflashovers from the round edges of the electrodes.

The applied HVDC-voltage was regulated according to the measured plaquethickness to reach to a mean electric field of 30 kV/mm. The temperaturewas 70° C. The current through the plaque was logged throughout thewhole experiments lasting for 24 hours. The current after 24 hours wasused to calculate the conductivity of the insulation.

DC conductivity method 4: Electrical conductivity of a 1.5 mm modelcable sample of the crosslinked test polymer composition as aninsulation layer and the crosslinked test semiconductive composition asa semiconductive layer and measured at 70° C. and 30 kV/mm mean electricfield

Model Cable Preparation:

Three layered cable cores were produced using a 1+2 construction on apilot scale CCV line. The conductor was made of copper and had an areaof 1.5 mm². The inner and outer semiconductive layers consisted of thesame test semiconductive composition comprising a crosslinking agent,which in the experimental part below was peroxide. The innersemiconductive layer was 0.7 mm thick, the insulation layer 1.5 mm thickand the outer semiconductive layer 0.15 mm thick. The cable cores wereproduced in two steps. In step 1 the cable cores were extruded using aline speed of 8 m/min without passing through a vulcanisation tube. Instep 2 the cable cores went only through the vulcanisation tube with aline speed of 5 m/min. The tube has two heating zones for dry-curing(crosslinking under nitrogen), each of 3 m, and the temperatures used onthese two zones were 400 and 380° C. respectively. This resulted infully crosslinked cables due to peroxide in the insulating andsemiconductive materials. The cooling section was 12.8 m long and thecable was cooled with water holding a temperature of around 25-30° C.

The cables were not degassed prior to conduction current measurements.To avoid unwanted degassing to occur the cables were covered withAluminium foil until the measurement were conducted. The cables werethen cut into 3 meter long samples with 100 cm active length(measurement zone) in the middle where the outer semiconductive layer ispresent. The outer semiconductive layer in the 100 cm ends of the samplehas been removed by a peeling tool. The schematic view of thethree-layer model cables with insulation thickness 1.5 mm used in method5 is illustrated in FIG. 1.

Conduction Current Measurements:

Conduction current measurements are performed by a three-terminal cellwhere the conductor acts as the high voltage electrode. The low voltageelectrode is an aluminium foil covering the outer semicon in the activepart. Guard electrodes are introduced by aluminium foil covering theinsulation on both sides of the measurements zone. The gaps between thelow voltage electrode and the guard electrodes are 5 cm.

Applied voltage is 45 kV DC (30 kV/mm mean electrical field) and thetemperature 70° C. The measurements are terminated after 24 h and theconductivity is measured as the average between 23-24 h. Thesteady-state current (the leakage current) is used in the calculations.

The conductivity s (S/m) has been calculated using the formula

$\sigma = \frac{\ln \left( \frac{D}{d} \right)}{2\pi \; {LR}}$

And R=U/I=Applied voltage (V)/leakage current (A).

TABLE Data used for the calculation of the conductivity from model cablespecimens. Parameter Value L Cable length (m) 1 d Inner diameter of 2.8insulation (mm) D Outer diameter of 5.8 insulation (mm) U Appliedvoltage (kV) 45

Method for Determination of the Amount of Double Bonds in the PolymerComposition or in the Polymer

A) Quantification of the Amount of Carbon-Carbon Double Bonds by IRSpectroscopy

Quantitative infrared (IR) spectroscopy was used to quantify the amountof carbon-carbon doubles (C═C). Calibration was achieved by priordetermination of the molar extinction coefficient of the C═C functionalgroups in representative low molecular weight model compounds of knownstructure.

The amount of each of these groups (N) was determined as number ofcarbon-carbon double bonds per thousand total carbon atoms (C═C/1000C)via:

N=(A×14)/(E×L×D)

were A is the maximum absorbance defined as peak height, E the molarextinction coefficient of the group in question (l·mol⁻¹·mm⁻¹), L thefilm thickness (mm) and D the density of the material (g·cm⁻¹).

The total amount of C═C bonds per thousand total carbon atoms can becalculated through summation of N for the individual C═C containingcomponents.

For polyethylene samples solid-state infrared spectra were recordedusing a FTIR spectrometer (Perkin Elmer 2000) on compression mouldedthin (0.5-1.0 mm) films at a resolution of 4 cm⁻¹ and analysed inabsorption mode.

1) Polymer Compositions Comprising Polyethylene Homopolymers andCopolymers, Except Polyethylene Copolymers with >0.4 wt % PolarComonomer

For polyethylenes three types of C═C containing functional groups werequantified, each with a characteristic absorption and each calibrated toa different model compound resulting in individual extinctioncoefficients:

vinyl (R—CH═CH2) via 910 cm⁻¹ based on 1-decene [dec-1-ene] givingE=13.13 l·mol⁻¹·mm⁻¹

vinylidene (RR′C═CH2) via 888 cm⁻¹ based on 2-methyl-1-heptene[2-methyhept-1-ene] giving E=18.24 l·mol⁻¹·mm⁻¹

trans-vinylene (R—CH═CH—R′) via 965 cm⁻¹ based on trans-4-decene[(E)-dec-4-ene] giving E=15.14 l·mol⁻¹·mm⁻¹

For polyethylene homopolymers or copolymers with <0.4 wt % of polarcomonomer linear baseline correction was applied between approximately980 and 840 cm⁻¹.

2) Polymer Compositions Comprising Polyethylene Copolymers with >0.4 wt% Polar Comonomer

For polyethylene copolymers with >0.4 wt % of polar comonomer two typesof C═C containing functional groups were quantified, each with acharacteristic absorption and each calibrated to a different modelcompound resulting in individual extinction coefficients:

vinyl (R—CH═CH2) via 910 cm⁻¹ based on 1-decene [dec-1-ene] giving E=13.13 l·mol⁻¹·mm⁻¹

vinylidene (RR′C═CH2) via 888 cm⁻¹ based on 2-methyl-1-heptene[2-methyhept-1-ene] giving E=18.24 l·mol⁻¹·mm⁻¹

EBA:

For poly(ethylene-co-butylacrylate) (EBA) systems linear baselinecorrection was applied between approximately 920 and 870 cm⁻¹.

EMA:

For poly(ethylene-co-methylacrylate) (EMA) systems linear baselinecorrection was applied between approximately 930 and 870 cm⁻¹.

3) Polymer Compositions Comprising Unsaturated Low Molecular WeightMolecules

For systems containing low molecular weight C═C containing speciesdirect calibration using the molar extinction coefficient of the C═Cabsorption in the low molecular weight species itself was undertaken.

B) Quantification of Molar Extinction Coefficients by IR Spectroscopy

The molar extinction coefficients were determined according to theprocedure given in ASTM D3124-98 and ASTM D6248-98. Solution-stateinfrared spectra were recorded using a FTIR spectrometer (Perkin Elmer2000) equipped with a 0.1 mm path length liquid cell at a resolution of4 cm⁻¹.

The molar extinction coefficient (E) was determined as l·mol⁻¹·mm⁻¹ via:

E=A/(C×L)

were A is the maximum absorbance defined as peak height, C theconcentration (mol·l⁻¹) and L the cell thickness (mm).

At least three 0.18 mol·l⁻¹ solutions in carbondisulphide (CS₂) wereused and the mean value of the molar extinction coefficient determined.

EXPERIMENTAL PART PREPARATION OF POLYOLEFINS OF THE EXAMPLES OF THEPRESENT INVENTION AND THE REFERENCE EXAMPLES

The polyolefins were low density polyethylenes produced in a highpressure reactor. The production of inventive and reference polymers isdescribed below. As to CTA feeds, e.g. the PA content can be given asliter/hour or kg/h and converted to either units using a density of PAof 0.807 kg/liter for the recalculation.

Inventive Example 1

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2576 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 4.9 litres/hour ofpropion aldehyde (CAS number: 123-38-6) was added together withapproximately 119 kg propylene/hour as chain transfer agents to maintainan MFR of 2.1 g/10 min. The compressed mixture was heated to 166° C. ina preheating section of a front feed three-zone tubular reactor with aninner diameter of ca 40 mm and a total length of 1200 meters. A mixtureof commercially available peroxide radical initiators dissolved inisododecane was injected just after the preheater in an amountsufficient for the exothermal polymerisation reaction to reach peaktemperatures of ca 276° C. after which it was cooled to approximately221° C. The subsequent 2^(nd) and 3^(rd) peak reaction temperatures were271° C. and 261° C. respectively with a cooling in between to 225° C.The reaction mixture was depressurised by a kick valve, cooled andpolymer was separated from unreacted gas.

Inventive Example 2

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2523 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 4.5 litres/hour ofpropion aldehyde was added together with approximately 118 kgpropylene/hour as chain transfer agents to maintain an MFR of 2.0 g/10min. Here also 1,7-octadiene was added to the reactor in amount of 23kg/h. The compressed mixture was heated to 160° C. in a preheatingsection of a front feed three-zone tubular reactor with an innerdiameter of ca 40 mm and a total length of 1200 meters. A mixture ofcommercially available peroxide radical initiators dissolved inisododecane was injected just after the preheater in an amountsufficient for the exothermal polymerisation reaction to reach peaktemperatures of ca 272° C. after which it was cooled to approximately205° C. The subsequent 2^(nd) and 3^(rd) peak reaction temperatures were270° C. and 253° C. respectively with a cooling in between to 218° C.The reaction mixture was depressurised by a kick valve, cooled andpolymer was separated from unreacted gas.

Inventive Example 3

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2592 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 4.9 litres/hour ofpropion aldehyde was added together with approximately 77 kgpropylene/hour as chain transfer agents to maintain an MFR of 1. 9 g/10min. The compressed mixture was heated to 163° C. in a preheatingsection of a front feed three-zone tubular reactor with an innerdiameter of ca 40 mm and a total length of 1200 meters. A mixture ofcommercially available peroxide radical initiators dissolved inisododecane was injected just after the preheater in an amountsufficient for the exothermal polymerisation reaction to reach peaktemperatures of ca 281° C. after which it was cooled to approximately208° C. The subsequent 2^(nd) and 3^(rd) peak reaction temperatures were282° C. and 262° C. respectively with a cooling in between to 217° C.The reaction mixture was depressurised by a kick valve, cooled andpolymer was separated from unreacted gas.

Inventive Example 4

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2771 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 5.3 litres/hour ofpropion aldehyde was added together with approximately 86 kgpropylene/hour as chain transfer agents to maintain an MFR of 0.7 g/10min. The compressed mixture was heated to 171° C. in a preheatingsection of a front feed three-zone tubular reactor with an innerdiameter of ca 40 mm and a total length of 1200 meters. A mixture ofcommercially available peroxide radical initiators dissolved inisododecane was injected just after the preheater in an amountsufficient for the exothermal polymerisation reaction to reach peaktemperatures of ca 281° C. after which it was cooled to approximately203° C. The subsequent 2^(nd) and 3^(rd) peak reaction temperatures were273° C. and 265° C. respectively with a cooling in between to 226° C.The reaction mixture was depressurised by a kick valve, cooled andpolymer was separated from unreacted gas.

Reference Example 1

Purified ethylene was liquefied by compression and cooling to a pressureof 90 bars and a temperature of −30° C. and split up into to two equalstreams of roughly 14 tons/hour each. The CTA (methyl ethyl ketone,MEK), air and a commercial peroxide radical initiator dissolved in asolvent were added to the two liquid ethylene streams in individualamounts. Here also 1,7-octadiene was added to the reactor in amount of24 kg/h. The two mixtures were separately pumped through an array of 4intensifiers to reach pressures of 2200-2300 bars and exit temperaturesof around 40° C. These two streams were respectively fed to the front(zone 1) (50%) and side (zone 2) (50%) of a split-feed two-zone tubularreactor. The inner diameters and lengths of the two reactor zones were32 mm and 200 m for zone 1 and 38 mm and 400 m for zone 2. MEK was addedin amounts of 205 kg/h to the front stream to maintain a MFR2 of around2 g/10 min. The front feed stream was passed through a heating sectionto reach a temperature sufficient for the exothermal polymerizationreaction to start. The reaction reached peak temperatures were 253° C.and 290° C. in the first and second zones, respectively. The side feedstream cooled the reaction to an initiation temperature of the secondzone of 168° C. Air and peroxide solution was added to the two streamsin enough amounts to reach the target peak temperatures. The reactionmixture was depressurized by product valve, cooled and polymer wasseparated from unreacted gas.

Reference Example 2

Purified ethylene was liquefied by compression and cooling to a pressureof 90 bars and a temperature of −30° C. and split up into to two equalstreams of roughly 14 tons/hour each. The CTA (methyl ethyl ketone(MEK)), air and a commercial peroxide radical initiator dissolved in asolvent were added to the two liquid ethylene streams in individualamounts. The two mixtures were separately pumped through an array of 4intensifiers to reach pressures of 2100-2300 bars and exit temperaturesof around 40° C. These two streams were respectively fed to the front(zone 1) (50%) and side (zone 2) (50%) of a split-feed two-zone tubularreactor. The inner diameters and lengths of the two reactor zones were32 mm and 200 m for zone 1 and 38 mm and 400 m for zone 2. MEK was addedin amounts of around 216 kg/h to the front stream to maintain a MFR2 ofaround 2 g/10 min. The front feed stream was passed through a heatingsection to reach a temperature sufficient for the exothermalpolymerization reaction to start. The reaction reached peak temperatureswere around 250° C. and 318° C. in the first and second zones,respectively. The side feed stream cooled the reaction to an initiationtemperature of the second zone of 165-170° C. Air and peroxide solutionwas added to the two streams in enough amounts to reach the target peaktemperatures. The reaction mixture was depressurized by product valve,cooled and polymer was separated from unreacted gas.

Reference Example 3

Purified ethylene was liquefied by compression and cooling to a pressureof 90 bars and a temperature of −30° C. and split up into to two equalstreams of roughly 14 tons/hour each. The CTA (methyl ethyl ketone,MEK), air and a commercial peroxide radical initiator dissolved in asolvent were added to the two liquid ethylene streams in individualamounts. The two mixtures were separately pumped through an array of 4intensifiers to reach pressures of 2100-2300 bars and exit temperaturesof around 40° C. These two streams were respectively fed to the front(zone 1) (50%) and side (zone 2) (50%) of a split-feed two-zone tubularreactor. The inner diameters and lengths of the two reactor zones were32 mm and 200 m for zone 1 and 38 mm and 400 m for zone 2. MEK was addedin amounts of around 201 kg/h to the front stream to maintain a MFR2 ofaround 0.75 g/10 min. The front feed stream was passed through a heatingsection to reach a temperature sufficient for the exothermalpolymerization reaction to start. The reaction reached peak temperatureswere around 251° C. and 316° C. in the first and second zones,respectively. The side feed stream cooled the reaction to an initiationtemperature of the second zone of around 185-190° C. Air and peroxidesolution was added to the two streams in enough amounts to reach thetarget peak temperatures. The reaction mixture was depressurized byproduct valve, cooled and polymer was separated from unreacted gas.

Semiconductive compositions for semiconductive layers of the model cablesamples Semicon 2: LE0550, commercial grade from Borealis with acetylenecarbon black, density 1100 kg/cm³, DC volume resistivity at 23° C. lessthan 100 Ωm and at 90° C. less than 1000 Ωcm (ISO3915), Hot Set Test(200° C., 0,20 MPa, IEC 60811-2-1): Elongation under load 25%,

Permanent deformation 0%. Gottfert Elastograph 1,2 Nm.

Experimental Results:

Mineral oil=Inventive examples 1-3: mineral oil based lubricant, ShellCorena E150, supplier Shell; Inventive example 4: mineral oil basedlubricant, M-RARUS PE KPL 201, supplier ExxonMobil

PAG=References: polyalkylkene glycol based lubricant, Syntheso D201N,supplier Klueber.

PA=propion aldehyde (CAS number: 123-38-6)

MEK=methyl ethyl ketone.

TABLE 1 Summary and components of the Polymer compositions CompressorPeroxide mmol lubricant —O—O—/kg used in the polymer Polymerpolymerisation AO composition, ADD composition process Comonomer (wt %)(wt %) (wt %) Reference PAG 1,7-octadiene 0.08 49.9 (1.35) 0.35 example1 Reference PAG No comonomer 0.19 77.7 (2.10) — example 2 Reference PAGNo comonomer 0.19 70.2 (1.9)  — example 3 Inventive Mineral oil Nocomonomer 0.19 66.6 (1.80) — example 1 Inventive Mineral oil1,7-octadiene 0.08 42.5 (1.15) 0.29 example 2 Inventive Mineral oil Nocomonomer 0.19 74.0 (2.0)  — example 3 Inventive Mineral oil Nocomonomer 0.07 48.8 (1.32) 0.26 example 4 AO: 4,4′-thiobis(2-tertbutyl-5-methylphenol) (CAS no. 96-69-5) Peroxide: Dicumylperoxide(CAS no. 80-43-3) ADD (Additive): 2,4-Diphenyl-4-methyl-1-pentene (CAS6362-80-7)

TABLE 2 Properties of the polyolefin components of the Polymercomposition Inv. Inv. Inv. Inv. Ref. Ref. Ref. Base Resin Properties ex.1 ex. 2 ex. 3 ex4 ex. 1 ex. 2 ex 3 MFR 2.16 kg, 190° C. 2.1 2.0 1.9 0.72.0 2.0 0.75 [g/10 min] Density [kg/m3] 922 920 921 922 922 922 922Vinyl [C═C/1000 C.] 0.37 0.56 0.25 0.26 0.25 0.11 0.11 Vinylidene 0.170.19 0.20 0.16 0.26 0.22 0.22 [C═C/1000 C.] Trans-vinylene 0.04 0.070.04 0.04 0.06 0.05 0.04 [C═C/1000 C.]

TABLE 3 Conductivity (fS/m) of 1 mm pressmoulded plaques of crosslinkedPolymer composition of the insulation measured at 70° C. and 30 kV/mmmean electric field (DC conductivity Method 1) By-products ReferenceInventive in sample example 1 example 2 No degassing 166 (fS/m) 67(fS/m) Degassed  77 (fS/m) 14 (fS/m)

TABLE 4 Conductivity of 0.5 mm pressmoulded plaques of crosslinkedPolymer composition of the insulation measured at 20° C. and 40 kV/mm.(DC conductivity Method 2) Code Polymer Compressor Conductivitycomposition lubricant Comonomer (fS/m) Reference PAG 1,7- 0.30 example 1octadiene Inventive Mineral oil No 0.20 example 1 comonomer InventiveMineral oil 1,7- 0.10 example 2 octadiene

The gel contents of the inventive example 2 was 78.2 wt % and the gelcontent of the comparative example 1 was 77.9 wt %, when measuredaccording to Gel content method as defined above under Determinationmethods.

TABLE 5 Conductivity (fS/m) of 1 mm pressmoulded plaques of crosslinkedPolymer composition of the insulation measured at 70° C. and 30 kV/mmmean electric field (DC conductivity Method 3) By-products ReferenceInventive in sample example 3 (fS/m) example 4 (fS/m) No degassing 160.1(fS/m) 65.6 (fS/m)

TABLE 6 Cable compositions and test results for 1.5 mm model cablesmeasured at 70° C. and 30 kV/mm mean electric field. (DC conductivityMethod 4) Compressor lubricant Semiconductive Polymer used in theComposition of composition polymerisation the inner of the process ofthe and outer insulation polymer composition semiconductive Conductivitylayer of the of insulation layer layer of the measured from model cableof the model cable model cable a model cable sample sample sample sample(fS/m) Reference PAG Semicon 2: 450.1 example 3 Acetylene black (LE0550)Inventive Mineral oil Semicon 2: 72.6 example 4 Acetylene black (LE0550)

What is claimed is:
 1. A crosslinked polymer composition comprising apolyolefin, wherein the crosslinked polymer composition has anelectrical conductivity of 150 fS/m or less, when measured at 70° C. and30 kV/mm mean electric field from a non-degassed, 1 mm thick plaquesample consisting of a crosslinked polymer composition according todirect current (DC) conductivity method (1) as described under“Determination methods”, wherein the polyolefin is a low densitypolyethylene (LDPE) homopolymer or low density polyethylene (LDPE)copolymer.
 2. The crosslinked polymer composition of claim 1, whereinthe polyolefin is obtainable by a high pressure process comprising: (a)compressing one or more monomer(s) under pressure in a compressor, usinga compressor lubricant for lubrication, (b) polymerising a monomeroptionally together with one or more comonomer(s) in a polymerisationzone, (c) separating the obtained polyolefin from unreacted products,and recovering the separated polyolefin in a recovery zone, wherein instep (a) the compressor lubricant comprises a mineral oil.
 3. Thecrosslinked polymer composition of claim 2, wherein the mineral oil ispresent at a maximum level of up to 0.4 wt % based on the amount of thepolyolefin.
 4. The crosslinked polymer composition of claim 3, whereinthe mineral oil is a white mineral oil which meets requirements givenfor white mineral oil in European Directive 2002/72/EC of 6 August 2002,Annex V, for plastics used in food contact.
 5. The crosslinked polymercomposition of claim 1, wherein the crosslinked polymer composition hasan electrical conductivity of 140 fS/m or less, preferably of 130 fS/mor less, preferably of 120 fS/m or less, preferably of 100 fS/m or less,preferably from 0.01 to 90 fS/m, more preferably from 0.05 to 90 fS/m,more preferably from 0.1 to 80 fS/m, more preferably from 0.5 to 75fS/m, when measured at 70° C. and 30 kV/mm mean electric field from anon-degassed, 1 mm thick plaque sample consisting of a crosslinkedpolymer composition according to DC conductivity method (1) as describedunder “Determination methods”.
 6. The crosslinked polymer composition ofclaim 1, which has an electrical conductivity of 0.27 fS/m or less,preferably of 0.25 f/Sm or less, more preferably from 0.001 to 0.23fS/m, when measured at 20° C. and 40 kV/mm mean electric field from adegassed, 0.5 mm thick plaque sample consisting of a crosslinked polymercomposition according to DC conductivity method (2) as described under“Determination methods”.
 7. The crosslinked polymer composition of claim1, further comprising an antioxidant(s) selected from the groupconsisting of sterically hindered or semi-hindered phenols, aromaticamines, aliphatic sterically hindered amines, organic phosphites orphosphonites, thio compounds, and mixtures thereof, preferably selectedfrom thio compounds comprising sulphur containing phenolicantioxidant(s), preferably selected from thiobishenol(s).
 8. Thecrosslinked polymer composition of claim 1, further comprising a scorchretarder(s) selected from the group consisting of allyl compounds, suchas dimers of aromatic alpha-methyl alkenyl monomers, preferably2,4-di-phenyl-4-methyl-1-pentene, substituted or unsubstituteddiphenylethylenes, quinone derivatives, hydroquinone derivatives,monofunctional vinyl containing esters and ethers, monocyclichydrocarbons having at least two or more double bonds, and mixturesthereof.
 9. The crosslinked polymer composition of claim 1, wherein thepolyolefin is a saturated LDPE homopolymer or a saturated LDPE copolymerof ethylene with one or more comonomer(s); or an unsaturated LDPEpolymer, which is selected from an unsaturated LDPE homopolymer or anunsaturated LDPE copolymer of ethylene with one or more comonomer(s).10. The crosslinked polymer composition of claim 1, wherein thepolyolefin is an unsaturated LDPE polymer, which is selected from anunsaturated LDPE homopolymer or an unsaturated LDPE copolymer ofethylene with one or more comonomer(s), and comprises a total amount ofcarbon-carbon double bonds/1000 carbon atoms of more than 0.4/1000carbon atoms, preferably the total amount of carbon-carbon double bondspresent in the unsaturated LDPE is the amount of vinyl groups,vinylidene groups and trans-vinylene groups, if present, more preferablythe unsaturated LDPE polymer contains vinyl groups and the total amountof vinyl groups present in the unsaturated LDPE is preferably higherthan 0.05/1000 carbon atoms, still more preferably higher than 0.08/1000carbon atoms, and most preferably higher than 0.11/1000 carbon atoms.11. The crosslinked polymer composition of claim 10, wherein theunsaturated LDPE copolymer is an unsaturated LDPE copolymer of ethylenewith at least one polyunsaturated comonomer and optionally with one ormore other comonomer(s), preferably the polyunsaturated comonomerconsists of a straight carbon chain with at least 8 carbon atoms and atleast 4 carbons between non-conjugated double bonds, of which at leastone is terminal, more preferably, said polyunsaturated comonomer is adiene, preferably a diene which comprises at least eight carbon atoms,the first carbon-carbon double bond being terminal and the secondcarbon-carbon double bond being non-conjugated to the first one, evenmore preferably a diene which is selected from C₈- to C₁₄-non-conjugateddiene or mixtures thereof, more preferably selected from the groupconsisting of 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene,1,13-tetradecadiene, 7-methyl-1,6-octadiene, 9-methyl-1,8-decadiene, andmixtures thereof, even more preferably selected from the groupconsisting of 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene,1,13-tetradecadiene, and any mixture thereof.
 12. The crosslinkedpolymer composition of claim 1, wherein the polyolefin contains vinylgroups in total amount of more than 0.20/1000 carbon atoms, still morepreferably more than 0.30/1000 carbon atoms.
 13. A crosslinked powercable, preferably a crosslinked direct current (DC) power cable,comprising a conductor which is surrounded by at least one layer,preferably by at least an inner semiconductive layer comprising a firstsemiconductive composition, an insulation layer comprising a polymercomposition and an outer semiconductive layer comprising a secondsemiconductive composition, in that order, wherein the at least onelayer, preferably the insulation layer comprises the crosslinked polymercomposition as defined in claim
 1. 14. A process for producing acrosslinked power cable, preferably a crosslinked direct current (DC)power cable, comprising a conductor surrounded by at least an innersemiconductive layer, an insulation layer, and an outer semiconductivelayer, in that order, wherein the process comprises: applying on theconductor the inner semiconductive layer comprising a firstsemiconductive composition, the insulation layer comprising a polymercomposition and the outer semiconductive layer comprising a secondsemiconductive composition, wherein the insulation layer comprises apolymer composition which comprises a polyolefin which is an LDPEhomopolymer or LDPE copolymer and a crosslinking agent, preferablyperoxide, more preferably peroxide in an amount of at least 35 mmol—O—O-/kg polymer composition, preferably of at least 36 mmol —O—O-/kgpolymer composition, 37 mmol —O—O-/kg polymer composition or more,preferably from 37 to 90 mmol —O—O-/kg polymer composition, morepreferably 37 to 75 mmol —O—O-/kg polymer composition, and crosslinkingat least the polyolefin of the polymer composition of the insulationlayer, optionally, and preferably, at least one, preferably both, of thefirst semiconductive composition of the inner semiconductive layer andthe second semiconductive composition of the outer semiconductive layerin presence of a crosslinking agent and at crosslinking conditions sothat the insulation layer comprises the crosslinked polymer compositionas defined in claim
 1. 15. A process comprising: (a) compressing one ormore monomer(s) under pressure in a compressor, using a mineral oilcompressor lubricant for lubrication, (b) polymerising a monomeroptionally together with one or more comonomer(s) in a polymerisationzone, (c) separating the obtained polyolefin from unreacted products,and recovering the separated polyolefin in a recovery zone, applying ona conductor an inner semiconductive layer comprising a firstsemiconductive composition, an insulation layer comprising a polymercomposition and an outer semiconductive layer comprising a secondsemiconductive composition, wherein the insulation layer comprises apolymer composition which comprises the polyolefin obtained in step (c)and a crosslinking agent, preferably peroxide, more preferably peroxidein an amount of at least 35 mmol —O—O-/kg polymer composition,preferably of at least 36 mmol —O—O-/kg polymer composition, 37 mmol—O—O-/kg polymer composition or more, preferably from 37 to 90 mmol—O—O-/kg polymer composition, more preferably 37 to 75 mmol —O—O-/kgpolymer composition, and crosslinking at least the polyolefin of thepolymer composition of the insulation layer, optionally, and preferably,at least one, preferably both, of the first semiconductive compositionof the inner semiconductive layer and the second semiconductivecomposition of the outer semiconductive layer in presence of acrosslinking agent and at crosslinking conditions so that thecrosslinked polymer composition has an electrical conductivity of 150fS/m or less, when measured at 70° C. and 30 kV/mm mean electric fieldfrom a non-degassed, 1 mm thick plaque sample consisting of acrosslinked polymer composition according to DC conductivity method (1)as described under “Determination methods”.
 16. A polymer compositionwhich is crosslinkable, wherein the polymer composition comprises apolyolefin and a crosslinking agent, and wherein the polymer compositionhas an electrical conductivity of 150 fS/m or less, when measured at 70°C. and 30 kV/mm mean electric field from a non-degassed, 1 mm thickplaque sample consisting of a crosslinked polymer composition accordingto DC conductivity method (1) as described under “Determinationmethods”.
 17. The crosslinked polymer composition of claim 1, whereinthe crosslinked polymer composition has an electrical conductivity of100 fS/m or less, when measured at 70° C. and 30 kV/mm mean electricfield from the non-degas sed, 1 mm thick plaque sample consisting of thecrosslinked polymer composition according to the direct current (DC)conductivity method (1) as described under “Determination methods”. 18.The crosslinked polymer composition of claim 1, wherein the crosslinkedpolymer composition has an electrical conductivity between 0.5 and 75fS/m, when measured at 70° C. and 30 kV/mm mean electric field from thenon-degassed, 1 mm thick plaque sample consisting of the crosslinkedpolymer composition according to the direct current (DC) conductivitymethod (1) as described under “Determination methods”.
 19. Thecrosslinked polymer composition of claim 1, which has an electricalconductivity of 0.25 f/Sm or less, when measured at 20° C. and 40 kV/mmmean electric field from a degassed, 0.5 mm thick plaque sampleconsisting of a crosslinked polymer composition according to the directcurrent (DC) conductivity method (2) as described under “Determinationmethods”.
 20. The crosslinked polymer composition of claim 1, which hasan electrical conductivity between 0.001 and 0.23 fS/m, when measured at20° C. and 40 kV/mm mean electric field from a degassed, 0.5 mm thickplaque sample consisting of a crosslinked polymer composition accordingto the direct current (DC) conductivity method (2) as described under“Determination methods”.